That isn't what he actually said, first off.
Vatican: Pope Francis Was Not Singling Out Donald Trump With 'Wall' Remarks
He was speaking generally to a question leveled at him by a reporter. Pope Francis is, apparently, fond of talking about "building bridges, not walls," so someone asked him a question about that in relation to Donald Trump.
He basically said
IF someone was
ONLY interested in building walls, not bridges, they would "not be Christian." However, he went on to clarify that it was "not Gospel," and that he "wasn't going to get involved" with the question of "to vote, or not to vote." He also said that he was going to give Donald the "benefit of the doubt," and assume that his position was a bit more complicated than that.
All of that's perfectly reasonable, as far as I'm concerned (this coming from someone who actually agrees Donald Trump is a pretty objectively terrible 'Christian,' mind you).
It seems that this latest situation is being exaggerated as well, from what I'm seeing. The "invitation" actually didn't come from the Pope at all, but the head of some largely autonomous college associated with the Vatican.