There are a few reasons why Christians are Christian. One of the main reasons is to do with Jesus and who he was and what he did. After all if he was who he said he was and performed those miracles, rose from the dead then it would be foolish to deny his claims about being the son of God and that the way to heaven is through him. What's a way to convince them that the gospels cannot be true?
Ask them why God is so homophobic in Genesis (Lot in Sodom) but finds it perfectly OK for that same Lot to impregnate his own daughters once the missus turned into a pillar of salt? A capricious God is one that you cannot depend on. He burns down Sodom for being zenophobic (Lot refused a nomad a good meal and some grousing in the goodie with the host's wife). The Christians are presentist in their thinking that 6,000 year old books have any application to life on the ground today.