I would go about this argument this way: We know that Christ himself did not write any of the Gospels. Someone, therefore, had to write the stories down and copy the letters of St. Paul after the Corinthians, or whoever, threw the originals away. By the time the bishops got together to work all of this out, the only copyists were church clergymen. If the churchmen got everything wrong, from the selection of the books to the copying of the texts, to the translation of the texts, to the doctrine, why believe they did anything right? The most we can prove that they got right was …what?
Right, a few Jewish books, that even Jews cannot agree how to interpret to this very day. But Jews agree (mostly) on one thing: Jesus was not the Christ.
If this is really what happened, why does anyone have any faith at all? Maybe the arians were right, and Christ was really just a very good man and nothing more.
Put the burden on your challengers: where to they begin to believe? And why? This is what our Lord did, when he asked the lawyers, “What say ye about the Christ?”