The "Jesus resurrection story came from pagan religions" claim is formed in a total vacuum, like most of the anti-Bible arguments. First, none of the alleged pagan resurrection myths remotely resemble Jesus. The idea that the Jesus resurrection story was based on them is idiotic for that reason alone. Second, there is the logical problem that the authors of the New Testament, with the possible exception of Luke, were Jewish and it's very unlikely they knew anything at all about those pagan religions, so they couldn't have been imitating them anyways. Then the idea that four different people would all choose to concoct a story based on these pagan religions is even more preposterous. When you make up theories in a vacuum though, logical problems like that are ignored.
I read that the Roman version of Mithra does bear a striking resemblance to the Biblical resurrection, however, Romans created their version of Mithra, which originated with the Persians I think, AFTER Jesus, and was obviously imitating the Bible, not the other way around.
What's also bizarre is that none of them can settle among themselves which pagan deity Christianity is allegedly copying from. Is it Mithras, or Ishtar? I've even heard of bizarre theory about Christianity being a rip-off of Buddhism.
It's a degree hypocrisy I can't overlook. You know the old saying that the simplest explanation is almost always right. There being a historical Christ is an infinitely simpler explanation than any atheist conspiracy theory involving pagan deities being copied by the early church.