The Emperor Hadrian wrote a letter indicating that Serapis and Christ were the same god. From Hadrian – “There those who worship Serapis are, in fact, Christians, and those who call themselves bishops of Christ are, in fact, devotees of Serapis. Even the Patriarch himself, when he comes to Egypt, is forced by some to worship Serapis, by others to worship Christ.“Atwill uses this assumption to support his published claims that Christianity was synthesized by a Roman Imperial cult. Considering his lengths at parsing Roman history and motives, it's quite funny he has made such a tremendous blunder. If you have been following my last post, "Serapis and Jesus: Ray Hagins is Wrong," you will be aware that this "Hadrian" quotation is actually widely known to be a forgery dating just before the fifth-century. Seriously, you can't read an introduction to the Historia Augusta (the source of Atwill's quotation) without immediately encountering the forged nature of the document. Here is one by Glanwill W. Bowersock in a proceeding volume of the Genevan conference on the Historia Augusta in which he continues to discuss the late fourth-century dating of the source:
The presence of fiction in the Historia Augusta is by now an established fact. The mischievous author of this work pretended, as we all know, to be writing historical biographies; but this pretense, though compelling the inclusion of genuine historical material, imposed no perceptible limits on his wit and invention.
Fooey! Kinda makes you look askance at everything else Atwell has to say, doesn't it?
 “The Aethioica of Heliodorus and the Historia Augusta,” in Historia Augusta: Colloquium Genevense II., ed. Giorgio L. Bonamente (Italy: Edpuglia, 1994), 43.