Just when it seems the 2016 presidential race could not get any stranger, it took another sharp turn on Thursday as Pope Francis questioned Donald Trump’s Christian faith during a flight home from Mexico – and the GOP candidate fired back, calling the Pope’s comments “disgraceful.”

The Pope, responding to a question aboard the papal plane about trumps promise to build a US – Mexico border wall, said, “A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not the gospel.”

Trump immediately responded, “For a religious leader to question a person’s faith is disgraceful,” he said.

Pope Francis weighed in just hours after he prayed at the Mexico – US border for migrants who died trying to reach the United States. The Pope did not definitively say Trump is not a Christian. Not having heard Trump’s plans independently, Francis said he’d “give him the benefit of the doubt.” But he said, “I’d just say that the man is not a Christian if he said it this way.”

The comments dramatically escalated what had been an otherwise cordial dispute between the two in recent days over immigration policy. Trump said in response he is a “proud” Christian and accused the Mexican government of using the pontiff for its own purposes. “The Pope 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. He doesn’t see how Mexican leadership is outsmarting Pres. Obama and our leadership in every aspect of negotiation,” he said. “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.”