If there were a human named Jesus, we can be quite sure he was not the son of god.
Here's a theory:
A young unmarried girl was raped and impregnated. In a time where a unmarried girl would be stoned if she were raped, she had to make up a story. So she says she was impregnated by god.
And she stuck to it.
The end.

@RebeccaD723 yes, duh, we know he wasn't the son of god. i didn't say he was.

but there's no "the end" about your theory. in fact your theory presumes that jesus existed at all, and that mary existed at all. that's kind of a big, unproven presumption. if we accept that he did, and she did, then it presumes she was raped and not just adulterous. the motivation to make up a story would be just as great. now, even if there was no mary and no jesus, later on SOMEONE believed these people existed, and paul was one of them. he is known to have gone to greece and greeks are known to have believed in gods raping human women and producing demigods, real sons of whichver god. and we also know that this expression, "a real son of god," existed at the time. so the theory that whether or not there was a jesus, paul told the greeks there was a jesus, used the expression "a real son of god" to mean that jesus was a pious man, got taken literally, and that the story of his divine birth began there rather than with a desperate lie from a mary who may or may not have existed. your theory is not impossible but it is by no means "the end" and it isn't even the likeliest scenario.

g