At this point in The Great Gatsby, Nick had become suspicious about Gatsby's background. His suspicions were confirmed by the revelations of the "ambitious young reporter" who had investigated and subsequently published his findings about the life history of Jay Gatsby, born James Gatz.
In the first-person viewpoint in which the book is written, readers are given insight into Nick's reaction to learning the truth about Gatsby's life story - that everything he thought he had known about Gatsby's past was completely fictiticous. Nick is coming to grips with the information that Gatsby did not acknowledge his biological parents as being related to him. "The truth was that Jay Gatsby of West Egg, Long Island, sprang from his Platonic conception of himself."
Nick is saying that Gatsby created himself to be the perfect son of a perfect father; since the only perfect father would be God, then Gatsby was the "son of God." Furthermore,
he must be about His Father's business, the service of a vast, vulgar, and meretricious beauty....to this conception he was faithful to the end.