Trump may believe he is a Christian, but if he's not (I doubt he is) then our course of action is to pray and be a good testimony to him. Frankly, many in the Church have been smug to him (Max Lucado, John Piper, etc.)
Calling Trump a false prophet, though, is beyond stupid. I mean come on. He's not claiming to be a Church authority, he's merely thinks he's a believer. He could be wrong, but our response shouldn't be hostility. Terrible testimony. I have a lot of people in my life who call themselves Christians but likely are not. I keep in contact and hope for open doors to minister to them and tell them the true gospel. You should have the same attitude toward Trump.