Pope Francis waded into the US presidential race last week with strong criticism of Republican frontrunner Donald Trump, insisting that building walls to keep immigrants out of one’s country was un-Christian. During an in-flight press conference on the way back from Mexico, Francis was asked about Mr Trump’s proposal that the United States extend a fence along the full length of the US-Mexico border. The reporter cited remarks by Mr Trump that the Pope was a “politician” and was being used by Mexican officials. “Aristotle defined the human person as homo politicus – [so] at least I’m a human person [in his eyes],” Pope Francis said. “As far as being ‘a pawn’,” the Pope said, “that’s up to you, to the people, to decide.” But one thing Pope Francis said he did know was that “a person who thinks only of building walls, wherever they may be, and not building bridges, isn’t Christian”. Asked if a Catholic could vote for such a candidate in good conscience, the Pope told reporters: “I’m not going to get mixed up in that. I’ll just say, this man is not Christian if he says this.” Mr Trump responded by saying: “For a religious leader to question a person’s faith is disgraceful.” He said the Mexican government was using the Pope as a “pawn” and was telling him only one side of the story about the US-Mexico border. “He didn’t see the crime, the drug trafficking and the negative economic impact the current policies have on the United States,” Mr Trump said. He added that if ISIS attacked the Vatican the Pope would wish that Mr Trump had been US president as that would have meant the end of ISIS. Later he appeared to backtrack, saying the Pope was “doing a very good job” and had “a lot of energy”.