There are many claims that Christianity - and Judaism and Islam - all share many similarities with earlier religions. They similarities are true in many cases. Of course, that doesn't prove anything one way or another. If I write a novel that reminds people very much of another novel written long ago, it doesn't prove plagiarism, but it certainly does raise the question. In the case of the Judeo-Christian-Islamic Creation myth and the Christian Jesus the Christ story, there are many similarities with other religions, other deities. That's about all we can say about it without crossing from speculation into belief.
It is also true that there are some atheists (and so-called humanists/secularists) who are actually anti-Christian, and they prove this by producing false claims about various pre-Christian religions and declaring them 'the same' as Christianity.
Bob knows about this - he recently passed on a graphic on FB that purported to show how Christianity stole this and stole that from various religions, all great ha-ha and hee-hee for all the anti-Christians, except that about 80% of it was not actually true. And that's kind of a problem for me. Point out flaws in Christianty? OK, no problem, it has them. Point out similarity to other pre-Christian religions? Hey, if they're there, they're there. Make crap up in an attempt to attack Christianity? OK, but at that point, let's stop pretending you're anything but an anti-Christian. Lie to yourself about being 'secular' and 'humanist' and 'not religious' all you want, but don't piss down my back and tell me it's raining. I know hate when I see it. FYI, the 'you' in the preceding is meant to be the generic 'you', not you personally. Just venting.
I don't even mind it when people are anti-Christian. I just wish they'd be honest about it. "Oh, I'm not anti-Christian, I'm an atheist." My dying ***, son. Your hatred is palpable, your attacks are focused. If you hate Christianity, good for you, but let's not pretend it's something else. That's cowardly.