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.

The pope’s remarks came during a wide-ranging, midair news conference in which he also waded into the question of whether the Roman Catholic Church should grant an exception to its prohibitions on abortion and birth control in regions where the Zika virus is causing a public health emergency, including in much of Catholic-dominated Latin America.

Researchers say pregnant women are especially at risk, noting that the virus may be responsible for a spike in cases of microcephaly, a condition in which newborns have unusually small heads and brains.