Pope Francis suggested on Thursday that Donald Trump is "not Christian" because of the Republican presidential candidate's plan to build a wall along the Mexican border.

Trump fired off a statement in response almost immediately that ripped the pope's comments as "disgraceful" and said the pope will think differently about him after the Vatican is attacked by the Islamic State.

"If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS’s ultimate trophy, I can promise you that the Pope would have only wished and prayed that Donald Trump would have been President," Trump's statement began.

Trump continued: "I will not allow Christianity to be consistently attacked and weakened, unlike what is happening now, with our current President. No leader, especially a religious leader, should have the right to question another man’s religion or faith."

Francis's remarks about Trump's Christianity came during his trip to the Mexico-US border, at which he highlighted the humanitarian crisis of the migrants trying to make it to America.

Asked by a reporter on Thursday if a "good Catholic" could vote for Trump, the pope criticized Trump's proposal for a wall along the border, but also said he would not be weighing in on the election.

"A person who thinks only about building walls, wherever they may be, and not bridges, is not Christian. This is not gospel," Francis said, according to ABC News.

He added: "As far as what you said about whether I would advise to vote or not to vote, I am not going to get involved in that. I say only that this man is not Christian if he said things like that. We must see if he said things in that way and in this I give the benefit of the doubt."

The escalating disagreement between the pope and Trump

This strange feud between the pope and Trump began earlier this week, when Trump dismissed the pope's visit to the border and argued that Francis didn't appreciate "the danger of the open border we have with Mexico."

"I think that the pope is a very political person," Trump said on Fox Business Network's Varney & Co.

The pope responded on Thursday by embracing the characterization, noting that political engagement is central to Aristotle's definition of what it means to be a human being.

"Thank God he said I was a politician, because Aristotle defined the human person as 'animal politicus.' So at least I am a human person," Francis said, according to ABC. "As to whether I am a pawn -- well, maybe, I don't know. I'll leave that up to your judgment and that of the people."

Trump's statement in full