But today's battle comes after Trump himself has spent days repeatedly questioning the faith of his biggest rival, Ted Cruz.
"I've never seen anybody that lied as much as Ted Cruz," Trump said at a press conference Monday. "And he goes around saying he's a Christian. I don't know, you're going to have to really study that.”
At a Tuesday rally in North Augusta, South Carolina, Trump said Cruz "holds up the Bible and then he lies about so many things."
"He says he is religious," Trump said of Cruz during yesterday's town hall in Bluffton, South Carolina. "No wonder I'm winning evangelicals by two to one."
All this followed a tweet from last Friday wondering about Cruz's evangelicalism:
How can Ted Cruz be an Evangelical Christian when he lies so much and is so dishonest? — Donald J. Trump (@realDonaldTrump) February 12, 2016
Pope Francis criticized the Republican candidate in a statement Thursday morning at the culmination of his trip to Mexico. Informed of some of the rhetoric Trump has used regarding immigrants and his proposal to build a wall across the U.S.'s southern border, Francis said Trump's actions were "not Christian."
"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 responded moments after the pope's comments were released. "No leader, especially a religious leader, should have the right to question another man’s religion or faith," Trump said in a written statement.