Pope Francis said Thursday that Donald Trump is “not Christian” if he intends to build a wall along the U.S.-Mexican border.

“A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian,” Francis said. “This is not in the Gospel.”

Trump immediately fired back, saying it is disgraceful for a religious leader to question a person’s faith.

“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 because this would not have happened,” Trump said.

Trump says he believes the Pope had no right to question his religion.

“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.”

He says the Pope only made the comment because he has “only heard one side of the story.”

“He didn’t see the crime, the drug trafficking and the negative economic impact the current policies have on the United States,” Trump said. “He doesn’t see how Mexican leadership is outsmarting President Obama and our leadership in every aspect of negotiation.”

Trump says he plans to build the wall because “illegal immigration is so rampant.”