I would say he's a "cultural Christian": affliliated with a denomination (Presbyterian), but not showing any overt evidence of regeneration or more than a superficial understanding of the faith; remember that time he said he'd never sought repentance for his sins?
I had said around 10 years ago that I thought Barack Obama was the most outwardly secular president in my lifetime (going back to Nixon; Carter, if I was actually old enough to pay attention). Today, I'd say that was Trump. Obama at least implied that the church he attended, liberal as it was, was important to him.