Donald Trump is calling Pope Francis "disgraceful" and "a pawn" for questioning his Christian faith.

During a news conference Thursday held during his flight from Mexico back to the Vatican, the pope answered a question about whether Catholics should vote for Trump, saying, "A person who only think about building walls, wherever they may be, and not building bridges, is not Christian."

The pope said he wouldn't advise Catholics on who to vote or not vote for, but repeated, "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."

Trump responded, saying "if and when" ISIS attacks the Vatican, the pope would have wished he were president.

Trump continued:

For a religious leader to question a person's faith is disgraceful. I am proud to be a Christian and as president 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. They are using the pope as a pawn, and they should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant.