Thrusting himself into the combative 2016 presidential campaign, Pope Francis said Thursday that GOP front-runner Donald Trump "is not Christian" if he calls for the deportation of undocumented immigrants and pledges to build a wall between the United States and Mexico.The Pope, who was traveling back to Rome from Mexico, where he urged the United States to address the "humanitarian crisis" on its southern border, did not tell American Catholics not to vote for Trump.But Francis left little doubt where he stood on the polarizing issue of immigration reform. "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not the gospel," the Pope told journalists who asked his opinion on Trump`s proposals to halt illegal immigration. Trump immediately fired back, calling Francis` comments "disgraceful." "No leader, especially a religious leader, should have the right to question another man`s religion or faith," he said in statement. Trump added that the government in Mexico, where Francis spent the past five days, has "made many disparaging remarks about me to the Pope.""If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS`s ultimate trophy, I can promise you that the Pope would have only wished and prayed that Donald Trump would have been president," Trump said. By Thursday evening, the GOP candidate had softened his tone. "I don`t like fighting with the Pope," Trump said at a GOP town hall debate in South Carolina hosted by CNN. "I like his personality; I like what he represents." He added that he thinks Francis` remarks were "much nicer" than the media reported and that the Pope had been misled by Mexican officials. Trump also said that the Pope has an "awfully big wall" himself at the Vatican. That may be true, Catholic priests said, but Vatican City also has an awfully big door. The tussle between Trump and Francis - two outsized personalities who seldom shy from speaking their minds - seems to have been building for some time. Before the Pope traveled to Mexico, Trump cast the pontiff as a political naif who "doesn`t understand the dangers" at the U.S.-Mexican border.Trump social media director Dan Scavino suggested the pontiff`s comments were hypocritical. "Amazing comments from the Pope- considering Vatican City is 100% surrounded by massive walls," he tweeted.During the wide-ranging press conference aboard the papal plane, Francis also seemed to suggest that contraception may be used to prevent the transmission of the Zika virus and praised Saint John Paul II`s "holy friendship" with a Polish woman. But it was his comments on Trump that seem sure to dominate the political conversation, perhaps handing a gift to Trump`s GOP opponents and opening Francis to criticism that his papacy is too partisan and his policies too liberal. Polls indicate that while Democrats adore the Pope, Republicans view him a little less favorably. Asked whether American Catholics should vote for Trump, Francis demurred. "As far as what you said about whether I would advise to vote or not to vote, I am not going to get involved in that. I say only that this man is not Christian if he has said things like that."The Pope appeared somewhat unaware of Trump`s exact stance on illegal immigration, though, saying that he would give him "the benefit of the doubt" until he had heard exactly what the billionaire businessman had said.