While it is technically true that there is as much acceptable-to-atheists "proof" of FSM's existance as there is of God's existance, there is one key difference.
Christ claimed, in all seriousness, to be the Son of God. And there are many eye witness accounts of him doing things that other people could not do. Most notably, there are multiple eye-witness acounts of him walking around and talking to people a few days _after he was murdererd_. Some of these accounts come from people who had much to lose from sounding like crazy people and nothing to gain from sharing their testimony.
Many people who were "taken in" by the claims of Christ and became his followers suffered tremendously.
What was the incentive for them to perpetuate a falsehood?
When reading the Bible and considering its truth claims, in my estimation, you must come away with one of these three broad conclusions:
1) the Bible is sufficiently fabricated to mean that most depictions of events are untrustworthy. Little in the book is relevant, from a truth perspective
2) Christ existed, and some of the depictions about what he did were accurate, but he was a magician and a tremendously good one, and was willing to commit his own life and the life of his friends to keeping his magician status secret.
3) Christ was actually exactly who he said he was: The Son of God. People begrudgingly beleive him because, despite mathematically improbable odds, he fulfilled the labyritnth of prophecies that greatly predate him, he performed many earthly miracles over the span of a few short years, and because he ultimately appeared physically to many people after he was publicly executed.
Forget for a moment whatever objections you have regarding the veracity of the claims about the claims. My point is that FSM has no such claims at all.
FSM may very well exist, but he/she/it/them/us hasn't convinced (fooled?) a bunch of people that he/she/it/them/us does.