As with just about anything humans argue, there are good and bad versions of the claim that Christianity must be false because other cultures have similar traditions/beliefs dating back earlier. (Gene Callahan’s post prompted me to write on this.)
For example, by itself I think it’s a decent argument for an agnostic to say, “There are lots of pagan myths about gods impregnating virgin human women who then give birth to superhuman offspring, so that makes me think the gospel accounts of Jesus are also myths.”
However, it is a terrible argument to say, “There are lots of pagan stories of a great flood in antiquity, and so I think the Genesis account is just a myth.”
If there really were a giant flood, then everybody alive after the fact must have been descended from the survivors. It would be odd if other cultures didn’t have accounts of it.
For another example of a terrible argument (though I must confess I probably used something like this when I was an atheist), I often see people saying, “Humans throughout history have believed in tens of thousands of deities. You think every single one of them is wrong except yours. Really?”
Again, if there is a living being who created the universe, then it should not surprise us that every culture pines to know more about this being. It would be evidence against the God of the Bible if the Hebrews were the only ones who had a notion of God.
If we find evidence that the Egyptians in 800 BC studied the stars–and moreover that they had many erroneous beliefs about them–does that make us doubt modern astronomy?