Preacher's Genesis has been one of the biggest mysteries in the AMC show's entire run, but where did it come from, what can it do, and how does its story end? After 4 seasons of blasphemy, mischief and vampiric circumcisions, Preacher finally came to an close with an eventful offering that neatly tied up the narrative's few remaining loose ends. The finale, appropriately titled "End of the World," also bade a fond farewell to Genesis, the entity that enters Jesse Custer in the very first episode and allows him to command others to obey his will, even beyond the realms of possibility.
As its name suggests, Genesis is the progenitor of Preacher's story - the very thing that forces Jesse Custer to question his faith and reality and embark on a journey that leads the character directly to God himself. Predictably, Genesis is also the reason Jesse is hunted by a number of dastardly villains over the course of Preacher's 4 seasons, as the forces of Heaven, Hell and Earth begin to pay attention to this previously unknown church leader from the American South. Information regarding Genesis is drip-fed throughout Preacher's narrative, but only in the final episode does the audience get a full picture of the hybrid entity that calls Jesse Custer's soul its home.
As revealed by Fiore and DeBlanc in Preacher's first season, Genesis is the forbidden offspring of a demon and an angel - a combustible combination that has never previously come into existence, but is currently the most powerful creation in the universe. However, it isn't until the final few episodes that Preacher reveals Genesis' father as the archangel Cassidy shared a cell with while captured by the Grail. The birth of Genesis explains why this angel, played by David Field, finds himself in such a dire situation and abandoned by his colleagues up in Heaven - a punishment for copulating with the enemy. After escaping with Cassidy, the archangel reconnects with his lover, a demon and the mother of Genesis, and Jesse recruits the parents to help protect him (and Genesis) in the upcoming battle.
After Genesis was discovered by Heaven, the angels tried to lock the abomination away but ultimately failed, and this is revealed as the trigger for God to abandon his holy throne. As shown in Preacher's final episode, God ran away because he himself is susceptible to Genesis' power. Not everyone is capable of possessing these abilities though, as most humans simply explode after being bestowed with Genesis, forcing it to find a new host. The qualification to hold Genesis is having the "perfect balance" between good and evil.
In terms of capability, Genesis works in conjunction with a being's soul. The host can only wield the power properly if their soul is complete and, conversely, the voice of Genesis doesn't work on someone whose soul has been removed - as proven with the Saint of Killers. This is likely why God can be manipulated using Genesis, because despite his lofty stature, even the Lord has soul. When making commands, Genesis is capable of performing acts that would otherwise be impossible, like travelling directly from Earth to Hell, or levitating in midair. The only stipulation is that the person involved must be listening and must understand the command. This is likely why God doesn't appear at Jesse's request when he and the pilot are stranded at sea.
In Preacher's finale, Jesse relinquishes control of Genesis and the creature shoots off into the night sky. This sacrifice is made because Jesse wants to trick God into feeling safe enough to return to Heaven, where the preacher has already arranged for the Saint of Killers to be waiting. Exactly what becomes of Genesis after this scene is unknown, especially since both its parents were killed during the fight against the Saint back in Masada. However, Jesse's final command was saying "be free" to Genesis, and the overriding implication is that the creature took this order literally, stopped looking for hosts to inhabit and went about its own business.