Don't bother, he's simply wrong. Pretty much any Church can claim legitimacy, and point to their interpretation of the Bible or other works for their support. As long as people are willing to believe their dogma, that is.

Meanwhile, given that one can construct arguments to make the Bible and other works fit pretty much any narrative you wish to support, isn't it all a bit pointless? I mean even you, a smart well educated person with probably rather better grasp of Latin slips up in a basic mistake of gender -- so what are the odds that the under-educated slaves and monks didn't mess up far worse through years of little slip ups filtered into Canon? And that's before one even considers the more difficult issue of contradictions...