God, also called the Creator, Lord, or the Man Upstairs, is widely known as the maker of the entire Universe. Upon his existence, he inhabited Heaven, and later created Archangels, Seraphs, Angels and Cupids, and they resided with him in Heaven for a long period before he left Heaven, and retreated to Earth, Castiel has become the new God in The Man Who Knew Too Much. It has been speculated that Chuck Shurley is God, though this has not been confirmed.
God is, by far, the most revered and powerful god in existence. (as his archangels are more powerful than the other gods) in history and is worshiped above all the other gods. He also created the planet Earth, Angels, humans and possibly the whole universe.
In the beginning, God alongside Death existed. He then created Heaven as a dwelling place where he and his angels will reside. He then created four Archangels named Michael, Lucifer, Raphael and Gabriel. He then created Angels and Cupids. The Angels and Archangels called God their father and existed only to glorify him.
Soon, God created humans. He then asked all the Angels to bow down to the humans as his greatest creations, and present more compassion towards the humans than to him. Lucifer soon became envious, as he was the favorite son of Heaven, and saw that humans were flawed and murderous (with both Death and Gabriel describing it as essentially being a "temper tantrum"). Faced with this situation, Lucifer became increasingly more and more prideful. He convinced one-third of all of Heavens Host to rebel with him. Soon before, Lucifer went to Michael asked him to stand with him, but Michael refused. A great and bloody battle then ensued in Heaven where Angel warred against Angel. Michael and Lucifer fought in a cataclysmic battle until eventually, God caused the battle to cease by casting all the rebels, including Lucifer, out of Heaven. Out of anger against god, Lucifer corrupted a human soul later known as Lilith into the first demon. God had Michael cast him into Hell (to be specific, Lucifer's Cage). God then continued to watch over and protect all his creation and children against anything that might hope to corrupt it, although it is inferred throughout the series that he is miserable, as he feels guilt for letting Lucifer be corrupted.
In "Sympathy for the Devil", Dean and Sam are trapped while Lucifer is about to emerge from his cage. Eventually, everything becomes bright and the brothers find themselves on an air plane. They were put there by none other than God, Himself. Later, when Dean questions Sam on how he's feeling, Sam notes that it's weird because he's not shaking and doesn't have a fever like he normally does after drinking demon blood. Sam gives the credit to whoever saved them and put them on the plane. In "The Dark Side of the Moon" it is confirmed by Joshua that God put them on the plane. As well as allowed both brothers into Heaven, resurrect Castiel and cured Sam.
Though it seems that he did that in order to have Joshua tell them to back off and that he is finished with his intervention. Which Joshua comments on being more than he has done in a long time. Joshua also comments that God may get lonely and since Joshua can sympathize with him that is why he gets to talk to him. Joshua also says that "you could drive yourself nuts asking questions like that" questioning some of his decisions. It is also speculated that Chuck Shurley is, in fact, God due to the fact that at the end of the season five finale, "Swan Song", Chuck evaporates after stating: "Nothing really ends".
In "The Third Man" Castiel states that God did not let Sam out of Hell, and since "Swan Song" no one has seen him (most likely even before as none of the angels knew where he was on Earth when he originally left Heaven.) Even Balthazar states "Dad, isn't coming back." and Raphael later references him when battling Castiel stating "Somehow I don't think God will be bringing you back this time."
In "The Man Who Would Be King", Castiel is desperate and morally ambiguous, and tells God the story of how he freed Sam from Lucifer's Cage, made a deal with Crowley and partnered up with him to find Purgatory, and started the Civil War in Heaven against Raphael. He prays to God to give him a sign, to which he receives no response.
In "The Man Who Knew Too Much," Castiel absorbs millions of souls to gain power, and snaps Raphael to pieces. He claims the souls as his own, and is adamant on not surrendering their power. When Sam arrives and stabs Castiel, the blade doesn't kill him, proving that he is not an angel anymore. He proclaims himself as the new God, and gives the Winchesters and Bobby an ultimatum: bow down to him or he will destroy them.
Powers & Abilities
Omnipresence: God exists everywhere in the universe as he is one of the two fundamental forces in it, the other being Death.
Omniscience: God is said to have unlimited knowledge and understanding and awareness of the universe. Yet he doesn't know whom is older, himself or Death. As Death has stated that neither can remember.
Omnipotence: God is said to be the most powerful being in the universe but it is not known if he is more powerful than Death. God can do anything he chooses and wants.
Immortality: God is older than time and possibly older than Death but neither remember who is older. In terms of immortality, he is also invincible to all kinds of harm whatsoever as well as never aging. It is possible that he can die as Death said that he will reap him in the future.
Teleportation: God was able to put Sam and Dean on a plane, when they were outside Lucifer's cage.
Resurrection: God was able to resurrect Castiel.