For what it is worth, my take on what Pope Francis said is that he was using the word "Christian" as an adjective, not as a noun. I think he was using it in the sense I am using it in the sentence below:
That is not a very Christian thing to do. In this sense, Christian could mean merciful, ethical or even nice in a sort of bland way.
I don't think the Pope meant it as a noun. For example, one might say:
Are you a Christian (noun)? No, I am a Zoroastrian (noun), not a Christian (noun).
When I went on dictionary.com and looked up the word "Christian" as an adjective, it listed several definitions. I think definitions 4-6 were what Pope Francis had in mind:
4.
exhibiting a spirit proper to a follower of Jesus Christ; Christlike:
She displayed true Christian charity.
5.
decent; respectable:
They gave him a good Christian burial.
6.
human; not brutal; humane:
Such behavior isn't Christian.
I don't think Pope Francis meant that Donald Trump does not have a personal faith in Christ. Not at all.