Warning: Contains spoilers for One-Punch Man chapter #156!
In One-Punch Man, the mysterious being known as "God" is slowly being set up as the main antagonist of the series. His true power and goals are still unclear, but what has been shown of God until now makes him a worthy opponent for the over-powered protagonist of the manga, Saitama. To get ready for that inevitable showdown, let's have a look at everything that has been revealed until now of God.
The character first appeared in chapter #109 of the manga, in a flashback detailing the origins of Homeless Emperor, one of the top executives of the villainous Monster Association. God is the entity that granted Homeless Emperor his powers, transforming him from a homeless man into a "Disaster Level: Dragon" monster who defeated many top-tier heroes. Later, God appeared in a vision had by Psykos while she was trying to merge with the "King of Monsters", Orochi. God appeared to her as a huge mass of brain cells, bestowing upon Psykos an even greater power and, more interesting, the sense of a greater destiny waiting for her. God finally appeared to Saitama, the S-Class hero Flashy Flash and the monster Oculette, when they touched a strange, cube-shaped object, and were able to hear God's voice in their heads. He offered them to make a "fervent wish", to see if they were "worthy vessels to bestow my power upon".
It appears that God, then, acts by bestowing power upon individuals that he either reaches out to or comes in contact with one of his cubes, described by the number one hero Blast as transmitters that distort space and allow for communication with "him". God's gifts, however, come with a price, something he calls "confiscation" and that he demonstrated in chapter #153 of the manga, where he also made his first full-scale appearance. When Homeless Emperor was being questioned by the hero Zombieman, he mentioned God as the source of his powers, but before he could reveal anything more, a colossal figure appeared on the Moon. Homeless Emperor then found himself in yet another vision, standing in an endless garden in front of a gigantic God, who took away his powers, along with his life.
It is still unclear what God's true goal is. He surely has a hidden agenda in tempting people with his powers, as shown in his latest appearance, in chapter #156, when he tried to trick S-Class hero Tatsumaki by taking the appearance of Blast. The number one hero also has a clear connection with the mysterious creature. He collects the cubes, trying to prevent people from touching them, and seems to be involved in a secret war that could be against God himself. Further details on God's final objective were revealed by Orochi. He discovered a mural in an ancient underground shrine, depicting Orochi and stating that a worthy sacrifice was needed in order to resurrect "God", leading the Monster King to believe this was referring to him. Soon after Orochi's defeat at the hands of Saitama, the ancient cave collapses, revealing the full picture of the actual God hidden in the mural.
If the writings in the ancient shrine are reliable, then God's purpose is to be resurrected: "Crossing the vast expanse of time, our god shall be resurrected on this Earth", if a worthy sacrifice is offered. Perhaps God is bestowing powers upon creatures as a way to look for the right sacrifice, or rather a "vessel", as he uses this exact word when communicating with Saitama's group. This is just a theory, however, but Blast seems to be the man who holds the answers to the mystery of God. Fans of One-Punch Man will have to wait to find out if this will be finally a worthy opponent for Saitama.