Couple of things
First, the pope didn't say "Donald Trump is not a Christian." He said, "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."
So, is he talking about Trump? Yes, probably. Also Jeb Bush, Ted Cruz, Ben Carson, Bobby Jindal, and, probably, Barack Obama. And a lot of other people.
The second thing? He didn't say "is not A Christian". He's not questioning Trump's religious affiliation, the way people, say, oh, Republicans, question the religious affiliation of, oh, say, President Obama. He's saying that when the goats are separated from the sheep, Trump's going to be in that crowd of people saying "What? When did we do that?" He's saying that actions speak louder than words, and that "not everyone who sayeth unto me 'Lord, lord' shall enter into the kingdom of heaven".
Straight up Christian ethical theology, in other words.
Saying Trump's policies aren't Christian isn't the same thing as saying that Trump is really a sekrit Mooslin. It's just pointing out that many American Republicans have a real trouble with the words of Jesus.
And that ain't news.