The Word of God was inscribed on a series of tablets that were dictated by God during creation "for the defense of mankind". Metatron is the Angel who recorded God's word on a series of tablets. Crowley described Metatron as "The scribe... and suck-up. Took down God's word, picked up His cleaning." Metatron's personal note on the demon tablet reveals that there is a compendium of tablets about a variety of subjects, presumably on all the creatures that God created.  Metatron disappeared after completing the Tablets. 
Castiel says that if the Word of God is revealed a prophet will be awakened - this prophet turns out to be Kevin Tran.
The first tablet in the series was excavated by Dick Roman in Iran and describes how to kill the Leviathan. The second tablet was in the hands of Crowley and Kevin Tran, and is titled 'Demons'. It includes instructions on how to construct a weapon against demons, information on how to see hellhounds and most importantly how to seal the gates of Hell by undertaking a series of Trials. Angel protocol dictates that the prophet be taken to a desert to learn the Word. In Kevin's case, he isolates himself on a boat owned by Garth.
Under torture by Crowley, the angel Samandiriel reveals that there is also a tablet relating to angels. Castiel locates it with Dean's help in one of Lucifer's Crypts and goes on the run with it to protect it from all threats after the tablet frees him from Naomi's control. It contains instructions on Trials that once completed will seal Heaven
Charlie manages to get into Dick's office and download his emails. Back at her desk she sends these, along with the contents of Frank's hard drive to the boys. She discovers that a package for Dick is arriving from an archeological dig in Iran at a nearby airport and manages to send a fake email, so it appears to be arriving 30 minutes later. This allows Sam and Dean time to get to the airport and switch the package for a case with a borax bomb in it.
When they open the case they find it contains a slab of rock.
Dean cracks the stone containing the tablet. As he does so there are electrical storms, women go into labour, Kevin Tran becomes a Prophet, and Castiel wakes from his comatose state. What can be read from the translation is of the following:
"Cut off the head, and the body will flounder. Waste not thy time nor your breath upon the Leviathan herd. Point thy blade at the heart of their master, for from him springs all their messages. Leviathan cannot be slain but by a bone of a righteous mortal, as light and good as the Leviathan are hungry and dark, washed in the three bloods of the fallen: a fallen angel, the ruler of fallen humanity, and the father of fallen beasts."
Sam and Dean decide to start gathering the needed parts of the weapon and start by getting the fallen angel blood from Castiel. When Kevin returns home and is captured by Edgar, he still has the tablet and it is captured with him.
After having captured Kevin Tran, Dick Roman forces him to translate the tablet for him. After the translation is done, Dick sends out Edgar to kill the Alpha Vampire and get his blood and summons Crowley into a Devil's Trap to make a deal with him.
At the same time, Bobby Singer figures out that the Ruler of Fallen Humanity is Crowley and that for a Father of Fallen Beasts they need an Alpha. Crowley is summoned and promises them his blood once they get the other needed pieces. Sam and Dean are able to get the Alpha Vampire to give them his blood after they save him from Edgar and prove that the Leviathans have betrayed him.
Sam and Dean steal the bone of a nun, Sister Mary Constant to get the "bone of a righteous mortal" while Dick makes a deal with Crowley to have him give them the wrong blood. However, Crowley informs them of the deal and gives them the right blood and they are able to make the weapon the tablet describes. Dean is later able to use it to kill Dick Roman with the help of Castiel.
After Crowley captures Kevin, he takes him to a warehouse where he shows him another Word of God tablet, this time about demons and has him translate it for him. Kevin learns it reveals how to throw open the gates to Hell and let demons out onto the Earth and directs him to one in Wisconsin while he performs a spell from the tablet to open it. However, Kevin casts another spell, this one from a section on how to kill demons and kills two demons holding him prisoner and escapes. Kevin hides the tablet somewhere, but learns how to protect himself from demons from it and that there's also a way to close the gates of Hell forever and banish all demons from the Earth.
After being stolen from the bus station where it was kept, Sam and Dean track it to a pawn shop where it was sold to Beau. The tablet is then sold at an auction run by the God of Greed Plutus. Sam, Dean, Kevin and Linda Tran team up to recover it while Crowley and the angel Samandiriel also try to get it. When the bid doesn't get high enough, Plutus tries to sell Kevin as well. Eventually Mrs. Tran bids her soul and the tablet is sold to her, Sam and Dean. However, Crowley possesses Mrs. Tran and teams up with Beau to betray and kill Plutus. In the fight that follows, Crowley, who learns of Sam and Dean's plan to close the gates of Hell forever, gets his hands on the tablet and escapes with it.
In a desperate attempt to translate the tablet, Crowley abducts all future Prophets, but none of them possess the capability to read it until Kevin dies.
Linda Tran comes up with a plan to strike back against Crowley by hiring a witch named Delta Mendota to gather the ingredients they need for the "demon bomb" spell Kevin used to escape Crowley's henchmen which are scattered all over the world. Delta brings them enough ingredients for at least one demon bomb, but betrays them to Crowley who kidnaps Kevin and leaves a demon to kill Linda. However, Linda captures the demon and takes him to Sam and Dean along with the ingredients and Kevin's notebook containing the information on how to create the bomb.
Crowley, even though he has all of the future prophets, prefers that Kevin translate the tablet for him rather than him kill Kevin and get one of the others to do it. After Crowley cuts off one of Kevin's fingers, he agrees. Kevin reads the titles of the various sections to Crowley which include the section on how to kill demons and how to seal the gates of Hell. However, Kevin also finds a note from Metatron, the writer of the tablet stating that after he writes it, he plans to take his leave of God and his brethren and leave the tablets for humanity, revealing that there is a compendium of tablets, interesting Crowley with the fact that there are tablets on more things than demons and Leviathans.
Sam, Dean and Castiel launch an attack on Crowley's warehouse to rescue the prophets and retrieve the tablet. During the rescue, Sam uses a demon bomb to kill three demons. Castiel confronts Crowley across a table that holds the tablet and when Castiel starts to use his powers on him, Crowley tries to grab the tablet and flee, but Castiel breaks it in half, leaving the escaped Crowley with one half and the Winchesters with the other. Sam and Dean decide to set out and find the remaining half of the tablet and Castiel chooses to join them.
Needing to deal with a lot of demons guarding the captive angel Samandiriel, Dean and Castiel travel to Kevin to get the ingredients for the demon bomb which they are out of. Kevin, who is trying in vain to translate the half of the tablet he has, is annoyed with them and reveals how difficult just the first three ingredients would be to get, however, Castiel asks for a list and heads off to get them. Kevin is shown to be running himself into the ground trying to translate the tablet and is very disheveled.
When being tortured, Samandiriel brings up the tablets, which is apparently hardwired into all angels and specifically the demon tablet which Crowley is uninterested in as he already knows about it.
Castiel is able to get the needed ingredients and during the assault on Crowley's headquarters, Sam and Dean are able to kill four demons with the demon bomb, killing two each with a bomb they each threw.
After weeks of non-stop work, Kevin finally manages to translate the part of the tablet about closing the gates of Hell. According to Kevin, the tablet reveals that someone must do three trials and read an Enochian spell after each one. Kevin translates it as "whosoever chooses to undertake these tasks should fear not danger nor death nor getting your spine ripped out through your mouth for all eternity." Once the three tests are done, the gates can be closed. Kevin is only able to crack the first test which is to kill a hellhound and bathe in its blood. Sam and Dean set out to kill a hellhound by looking for someone who has made a deal which is to come due soon.
Later, Kevin finds information on hellhounds and calls Dean to tell him about it, telling him it reads "the dire creatures may be seen only by the damned or through an object scorched with Holy Fire." Using this information, Sam and Dean come up with the idea to use glasses scorched by Holy Fire to see the hellhound.
In the end, Sam manages to kill a hellhound with Ruby's Knife and casts the spell.
After finding the angel tablet, Dean asks Castiel if he is now going to go back to looking for the demon tablet like he'd earlier claimed, but under Naomi's orders, Castiel tries to kill him instead.
Kevin calls Sam and Dean to tell them that along with Crowley apparently haunting his dreams, he has deciphered the second trial to close the gates of Hell. Kevin reveals that Sam must rescue an innocent soul from Hell and "deliver it unto Heaven."
Becoming ever more paranoid, Kevin hides his half of the tablet somewhere while Crowley figures out what the Winchesters are up to as they got the "good half" and he got the half with useless information on it.
Sam is able to rescue the soul of Bobby Singer with help from rogue Reaper Ajay and Benny Lafitte, but when he tries to release him to Heaven, Crowley intervenes and traps Bobby's soul, intending to take him back to Hell and prevent the completion of the trial. However, Naomi, wanting the trial to succeed, drives Crowley off and breaks his hold on Bobby's soul, allowing Bobby to ascend to Heaven and completing the second trial. Sam then casts the spell to signal the completion.
Having learned Kevin's location from Linda Tran's smartphone, Crowley captures Kevin and all of his notes on the tablet, leaving the Winchesters with no clues on the third trial and Crowley with everything Kevin has deciphered so far.
Kevin is captured by Crowley and held in an illusion of Garth's houseboat with all of his notes on the tablet. Two demons pretending to be Sam and Dean visit him with Crowley's half of the tablet to translate, claiming that they managed to get it from him. Kevin starts to work on translating the second half of the tablet, excited as it will hold the third trial.
At the same time, the real Sam and Dean receive a prerecorded message from Kevin with his notes and translations of his half of the tablet, sent in case something happened to him. While they are unable to figure out the notes anymore than Kevin could, a symbol seen repeatedly in the notes ultimately leads them to Metatron, the angelic scribe who wrote the tablets in the first place.
Eventually, Kevin realizes that he has been captured and tricks the demons pretending to be Sam and Dean into thinking they are going after his half of the tablet while in reality he sends them into a Devil's Trap. Crowley confronts Kevin who refuses to translate the tablet for him for real, saying he won't break this time and Crowley has no idea the power the tablet could give him. As Crowley now has the angel tablet he doesn't care and strangles Kevin, but Kevin is rescued by Metatron before Crowley kills him and takes with him the second half of the tablet.
At the casino where Metatron hides out, Kevin reveals Crowley's half of the tablet and tells Sam and Dean he has translated the third trial. Metatron reveals what it is and Kevin confirms it: they must cure a demon.
As he is killing Sheriff Jody Mills, the Winchesters call Crowley and he offers a deal to stop killing people they've saved: stop the Trials and turn over the whole demon tablet. Dean agrees, but only if Crowley trades it for the angel tablet. In order to make the deal happen, Kevin digs up the first half of the tablet where he hid it, ironically under a sign depicting the Devil. Kevin puts the tablet back together and gives it to Sam and Dean. At the ruins of Singer Salvage Yard, Sam shows Crowley the demon tablet to prove he has it while Crowley shows off the angel tablet. However, the deal is a trap and they capture Crowley. Sam begins work on curing Crowley to complete the trials, but stops after Dean learns from Naomi what the tablet didn't reveal: the person that completes the trials will die if they complete them as the ultimate sacrifice to seal the gates of Hell.
Samandiriel, who due to the spikes in his head has gone back to "factory settings," is tortured for anything useful that he knows. He speaks about the tablets which is apparently hardwired into all angels and before Dean, Sam and Castiel finally break in, reveals the existence of the angel tablet which interests Crowley who flees as the three break in.
After Samandiriel reveals that he told Crowley about the tablet, Naomi forces Castiel to kill him for revealing its existence. When questioned, she calls Samandiriel a traitor for revealing its existence as it is something that all angels would die to prevent demons from finding out about. Naomi reveals that like the demon tablet reveals how to seal all demons in Hell, the angel tablet may have something similar for angels on it.
Having discovered that Lucifer had the angel tablet, Crowley starts searching for Lucifer's Crypts using Meg who knows their locations from her time with Azazel. Meg only gives general locations of the crypts so Crowley has his demons posses people who might know where they were located and search for them while Castiel tortures and kills the demons responsible trying to get the tablet for Naomi. Eventually after capturing a demon, she leads them to Meg who is able to show them the correct location of Lucifer's crypt in the town which is where the tablet is.
While Meg and Sam defend the outside of the building where the crypt is from Crowley and his minions, Dean and Castiel search the crypt itself. Castiel is able to locate the correct box containing the tablet as it is warded against angels and has to tell Dean as he is the only one able to touch it. Dean finds the tablet encased in a rock similar to the Leviathan tablet and Castiel, on Naomi's orders, attacks him with an Angel Sword, trying to kill him to get the tablet. In the scuffle, the rock hits the ground and breaks, releasing the tablet. Eventually Castiel regains control over himself and when he picks up the tablet, it and he glow, severing his connection with Naomi and breaking her hold over him. After healing Dean, Castiel leaves with the tablet, telling Dean that all he knows is he must protect the tablet from Naomi, Crowley and even Dean for some reason.
Both Naomi and Crowley meet up in the crypt, foiled in their efforts to get the tablet, but Naomi is happy as Castiel is protecting the tablet as he is meant to, even if it means protecting it from her. Castiel is later seen traveling on a bus through the countryside with the tablet.
Naomi, Ion and Esper corner Castiel in a Biggersons and demand the tablet. Castiel refuses to tell them where to look and Naomi has the other angels search all the Biggersons for it, figuring Castiel hid it in one as he has been using them to hide. However, they are unable to find it and Crowley shows up, kills the other angel and drives Naomi off. Crowley has realized that since the tablet freed Castiel's mind, he wouldn't risk being parted from it and has it inside him. After shooting Castiel in the gut, Crowley digs the tablet out of him and leaves with it when he receives word that Kevin Tran has figured out his deception.
When visiting Kevin in his illusion of Garth's houseboat, Crowley brings the tablet with him. He no longer cares that Kevin won't translate the demon tablet or help him as he has the angel tablet and this causes him to try to kill him before Metatron rescues him.
Metatron approaches Castiel to talk about the situation in Heaven. He's done some investigating and found many factions all at war trying to gain control -Naomi leads just one of them. Castiel feels responsible after all the destruction he wrought in Heaven. Metatron invites Castiel to join him for some crepes at a restaurant. He tells Castiel that he thinks they should close Heaven off from Earth - to force the angels to sort out their issues, and also to prevent any of their conflict from causing damage on Earth. He says that the angel tablet contains trials which when completed will close Heaven. As the Scribe of God, Metatron knows them without the tablet. Metatron says he's just a writer, but that Castiel is a warrior.
Castiel agrees to undertake the trials - even though Metatron warns that they will be more difficult than Cas can imagine. He then points out Jane, the young woman who has been their waitress. Metatron reveals the first trial is to kill her and cut out her heart becasue she is Nephilim and considered an abomination. That night, the two angels trail her to kill her, but she reveals her knowledge of what they are and begs them not to kill her. As she is an innocent woman, Castiel hesitates, but when she attacks him and Metatron, Castiel kills her with his Angel Sword while she attacks Metatron and completes the first trial.
While threatening the life of Sheriff Jody Mills, Crowley demands the demon tablet as part of his deal to stop killing people Sam and Dean have saved and Dean only agrees if Crowley gives them the angel tablet in exchange. Crowley agrees and they meet in Singer Salvage Yard to make the trade and seal the deal. Crowley shows that he did in fact bring the tablet after Sam shows the demon tablet and demands that Sam sign a giant contract to seal the deal. Dean pretends to examine the contract and gets close to Crowley to slap him with handcuffs with a Devil's Trap painted on them, capturing him. With that, Sam and Dean get the angel tablet.
After learning that Castiel is trying the Trials to seal up Heaven, Dean brings the angel tablet to Kevin Tran to translate for the third trial while he and Castiel go to complete the second trial, getting the bow of a Cupid. Kevin is reluctant as he wants a normal life and it took him six months just to translate half of the demon tablet, but agrees after Castiel gets rough with him. Dean and Castiel succeed when the Cupid is convinced to give her bow to them willingly, but Kevin calls and says that while he has found trials to close Heaven on the tablet, he doesn't see anything about killing a Nephilim or getting the bow of a Cupid. Naomi arrives and claims that she has learned from Metatron that it is actually a lie: he doesn't want to seal Heaven, he wants to expel all angels from it to Earth in revenge for being expelled himself. She earnestly tries to convince Castiel, but while Dean believes her, Castiel doesn't. Kevin, who heard the whole thing over the phone is unable to confirm what Naomi said, but Metatron himself later does after killing Naomi and capturing Castiel.