ABOARD THE PAPAL PLANE — Thrusting himself into the heated American presidential campaign, Pope Francis declared Thursday that Donald Trump is “not Christian” if he wants to address illegal immigration only by building a wall along the U.S.-Mexico border.

Trump fired back, saying it was “disgraceful” for a religious leader to question a person’s faith.

The rare back-and-forth between pontiff and presidential candidate was the latest astonishing development in an American election already roiled by Trump’s free-wheeling rhetoric and controversial policy proposals, particularly on immigration. It also underscored the popular pope’s willingness to needle U.S. politicians on hot-button issues.

Francis’ comments came after he ended a visit to Mexico, where he prayed for people who died trying to reach the U.S.