Actually, both the Pope and the Donald have shown class by moving on beyond this. Both have backtracked. It is now a non-issue. We all should follow their example and put this issue behind us.
For Catholics, the Pope questioning Trump's Christianity may certainly be an issue for them since Trump has repeatedly declared himself a Christian.
If someone declares that they are a Christian, I accept it. However, when they make that declaration over and over in political rallies, I ask myself in what ways has he shown himself a Christian, turning the other cheek, spreading the gospel, doing unto others what you would have them do unto you.