ABOARD THE PAPAL AIRLINER -- Inserting himself into the Republican presidential race, Pope Francis on Wednesday suggested that Donald J. Trump "is not Christian" because of the harshness of his campaign promises to deport more immigrants and force Mexico to pay for a wall along the border. "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said when a reporter asked him about Mr. Trump on the papal airliner as he returned to Rome after his six-day visit to Mexico.