The pope was just 'riffing' on Trump: Professor

For all of Donald Trump's talk about his Christian beliefs, his faith is still in question, theology professor Chad Pecknold said Friday.

"Mr. Trump says he's a Christian? Tell us how the biblical principles guide him," the Catholic University of America professor told CNBC's "Squawk Box." "I think that's a legitimate question no one has asked."

Pecknold made his remarks a day after Pope Francis said the GOP front-runner was "not Christian" because of his views on immigration.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said aboard the papal plane on his way home from Mexico. He was answering a specific question about Trump's views.

In response, Trump said the pope would wish for a Trump presidency if the so-called Islamic State ever attacked the Vatican.

Pecknold said: "I think the pope is good at sending out grenades on these papal plane interviews, which are not authoritative teaching, they are just the pope riffing off events and answering questions."