Republican presidential frontrunner Donald Trump said Friday that Pope Francis did not understand the gravity of the “crime problem” caused by illegal immigrants crossing the US-Mexican border when the pontiff appeared to criticize Trump’s plan for a border wall.

Trump, in an interview with Fox News’ Greta Van Susteren, also maintained his stated opposition to the war in Iraq, which he has used in recent days to hammer presidential primary opponent Jeb Bush.

“He’s a terrific person,” Trump said of the pope, the leader of the world’s 1.2 billion Catholics. “To me it’s [about] illegal immigration. I don’t think the pope really understood in terms of the crime problem and the problems of illegal immigration.”

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Pope Francis said Thursday. A Vatican spokesman clarified the Pope’s remarks Friday, saying he “in no way [made] a personal attack” on the New York businessman.

"Without borders we don't have a country, and that [Mexican] border is just like a piece of Swiss cheese; people pour over," Trump told Van Susteren on "On the Record."

Trump also discussed a 2002 clip from the Howard Stern radio program in which he appeared to endorse the United States’ invasion of Iraq. He asserted he was only a supporter of action against Iraq “long before the war started.”

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“By the time the war started, I had gone—I was absolutely opposed to it. I always thought it was a destabilization of the Middle East,” Trump said.

“It was the worst decision ever made in the history of our country,” Trump said. “[The war cost] $2 trillion and thousands of lives.”

“Had [Iraqi dictator] Saddam Hussein lived—he was a bad guy, but he killed terrorists,” Trump said, adding that if former President George W. Bush “went to the beach and gone swimming [instead of engaging with Iraq], we’d be better off.”