He’s not an atheist; he has fixed upon a very peculiarly parochial idea of God, and of course all those ordinary Christians are just washed up and he’s the smartest person in the world.
I usually give a synopsis of the Old Testament and how it led to the New Covenant. Then focus on the New and what it really means to us. Some folks refuse to be convinced, but the upshot is that it does me a world of good to engage.