A few days ago I watched this video (see below). It attempts to explain something that the early Christian Apologist Justin Martyr said. You see, Justin was living in the second century, and he made a pretty damning statement about Christianity (which you'll see in the video). He points out that Christianity has quite a bit of specifics in common with older pagan religions. So the Christian in the video counters this by saying (basically): "Oh sure, he said it, but he only said it to show that the Romans were being inconsistent for NOT punishing other religous groups who believed the same type of stuff."
I might point out that it doesn't really matter why Justin said what he said, it only matters that he said it. If he said that Christianity had a lot in common with older religions (so much so that he attributes this to demons counterfeiting Christianity in advance) then why he said that DOES NOT MATTER. He still said it.
So the guy made this video goes on to give another quote from Justin Martyr that I think is worth addressing:
"[W]hatever we assert in conformity with what has been taught us by Christ, and by the prophets who preceeded him, are alone true, and are older than all the writers who have existed..."
So is Justin saying that Christianity is older than all these pagan poets (the 'writers')? Yes and No. Justin seems to be saying that the Old Testament prophets are older than the pagan poets. He is probably not saying that Jesus predates the pagan poets, because in chapter 23 of Justin's First Apology he says that before Jesus became a man among men, demons influenced the pagan poets to imitate Old Testament prophecies as having already happened to a pagan deity. So Justin realized that the pagan parallels needed an explanation, and he did the best he could to give one. That tells us something right there: If the parallels were not signifigant, he would not have needed to explain them. One could argue that Justin was wrong about these parallels needing an explanation, but look again at the parallels Justin cites in chapters 21-23 of his Apology (linked above) and you tell me if those parallels are insignificant nothings.
Also, there are several reasons Justin Martyr's explanation will not work. First, it smacks of special pleading because in any other case we would simply say that Christians borrowed this stuff from the pagans. Second, a lot of the stuff about Jesus that Justin thinks the Jews prophesied actually weren't: The Old Testament does not predict that the messiah would be born of a virgin, or that the messiah would be the literal son of God, that he would suffer a miserable death, be resurrected, heal the lame or blind, etc.