An Earthman who completed a course of study on an alien planet was given the ability to make worlds, which involved going through a ritual and being assigned a god of their pantheon. The ability to create worlds is dependent on his connection to this god.
When the book starts, he has been residing on his own planet for a while, either taking a break from or retired from worldmaking. [Creating custom planets pays very well.] He receives a notice from the aliens who gave him the ability to create worlds. Someone of his race -- he is the only Earthman to receive the gift of world creation -- has illegally gone through the ritual that gave him his power. The god this person was given is the sworn adversary of his god. Think of this as a god's version of a Holmes-Moriarty competition.
To protect the worlds he has created, he reluctantly leaves his planet and searches for his god's adversary who, as it turns out, is also his Earthly rival.
My memory insists the author was Roger Zelazny but I am not certain of that.