Angels in Supernatural
In Season 2, episode 2.13 Houses of the Holy Sam thinks they might actually encounter an avenging angel, who goes after the wicked, although Sam and Dean later find out, that the supernatural being Sam thought for an angel was in fact the ghost of a priest. Dean makes it known in the same episode that he doesn't believe in angels. This was the second time Supernatural takes on the question of faith and belief (see also 1.12 Faith.
Dean learns that angels are real, just as demons, when he encounters Castiel, an "Angel of the Lord", the very same being that pulled him out of hell and reawakened him. Castiel also reveals that the reason God wanted Dean out of Hell is that God has work for him to do.
Castiel appears to Dean in Bobby's kitchen, and confirmed that the spirits who attacked them were Witnesses. He tells Dean that their summoning by Lilith broke the first of The 66 Seals , and that when the last one is broken Lucifer will be freed.
Castiel has the ability to send Dean back in time, but apparently past events can't be changed.
An angel is an ethereal creature who assists and serves God or the gods of many religious traditions. In the Christian bible, angels are portrayed as powerful and dreadful, endowed with wisdom, correct in their judgment, holy, but not infallible. When their duties are not punitive, angels are beneficent to man. When their duties are punitive, they are known as avenging angels and are mentioned inverses such as II Sam. Xxiv. 15, in which an angel annihilates thousands on God's command. These avenging angels are used by God to punish men for their sins.
<P>A supernatural being found in Christianity, Judaism, Zoroastrianism and Islam. Angel comes from the Latin word angelus, which came from the Greek word, angelos, meaning "messenger". According to these religions Angels typically act as messengers from God.
Though superhuman, angels can assume human form and according to the Hebrew Bible, often appeared to people in the shape of humans of extraordinary beauty. They are also described as pure and bright and are said to be formed of fire, and encompassed by light. Angels are thought to posess wings, and are depicted that way in Christian, Jewish and Zoroastrian art where they are also commonly depicted with halos.
In the Bible, angels are a medium of God's power; they exist to execute God's will. Angels reveal themselves to individuals as well as to the whole nation, to announce events, either good or bad, affecting humans.</p>
What the creators have to say about Angels in Supernatural
Never say never, (...) but in my mind, angels as supernatural beings do not exist. In my opinion there are forces of evil, but the forces of good are human. There might very well be a God, but if there is, he's working in mysterious ways through a band of very imperfect humans.