Michael mentions that while talking with Lilith
After the creation of [[Heaven]], its [[angel]]s, and Earth, God commanded all of the celestial beings to bow before mankind, asking them to love humans more than himself. Lucifer questioned this order, insisting that they are "flawed, murderous beings."<ref name="five04">[[5.04 The End]]</ref> Lucifer then sought out his older brother, Michael, for support, but Michael refused to listen. After Lucifer created the first demon, [[Lilith]], Michael would be the one to cast Lucifer out of Heaven into [[Hell]]. At some point, God left Heaven and Michael assumed command of the angels, running Heaven and Earth for a millennia.
When [[demon]]s began making plans to free Lucifer, Michael decided to allow the seals binding Lucifer to be broken. He and a group of other angels, including Raphael and [[Zachariah]], wanted the [[Apocalypse]] to occur so that he and Lucifer could battle. Michael expected to be victorious, so as to bring about paradise. Lesser angels, like [[Castiel]], were not privy to the plan, and were deployed to make it look as though Heaven was fighting Lucifer's release. When Lucifer was eventually freed, in order to confront him, Michael needed his true [[vessel]], [[Dean Winchester]]. Zachariah was instructed to get Dean's permission to possess him. To Heaven's chagrin, Dean refused adamantly, and Michael was left to act through his agents. He then decided to speak with Dean in person, when the Winchester brothers were sent back to the year 1978, and used their father [[John Winchester]] as a temporary vessel.
Over time, Dean's resolve weakened, and just as he prepared to give his consent, the angels resurrected his half-brother, [[Adam Milligan]], for the purpose of fulfilling the task of being Michael's [[vessel]]. This was soon revealed to be a ploy to make Dean give in, but when Dean killed Zachariah, after Michael had been called down from Heaven it forced Michael to take over Adam's body.