Pope Francis discussed Donald Trump during his in-flight press conference on the way back to Rome from Mexico

As the plane carrying him back to Rome from Mexico was flying over Texas, Pope Francis insisted building walls to keep immigrants out of one’s country is un-Christian.

Holding his customary in-flight news conference on Thursday after a six-day trip that ended at the Mexico-US border, Pope Francis was asked about his reaction to US presidential candidate Donald Trump’s proposal that the United States extend the fence along the full length of the border and his comments to Fox Business Network that Pope Francis is a politician and is being used by Mexicans.

“Aristotle defined the human person as ‘animal politicus’ — (so) at least I’m a human person” in Trump’s 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” about building walls.

Pope Francis spent an hour answering questions, including about contraception and the Zika virus, the recently publicised letters between St John Paul II and a woman philosopher, the sex abuse scandal and the reaction of Ukrainian Catholics to the joint declaration he signed with Russian Orthodox Patriarch Kirill of Moscow last Friday in Cuba.

A reporter asked why he had spent so much of the week denouncing the ills that plague Mexico, but said nothing of the scandal of clerical sexual abuse and, in particular, about the late Fr Marcial Maciel Degollado, the Mexican founder of the Legionaries of Christ. The priest lived a double life, fathering children and sexually abusing numerous seminarians.

Pope Francis’ comment on Fr Maciel’s case was to praise Benedict XVI who, as a cardinal and prefect of the Congregation for the Doctrine of the Faith, tenaciously investigated the allegations and insisted “there was a need to clean the Church’s dirt, the garbage.”

The Catholic Church has done much in the past dozen years to protect children, he said, but the work is ongoing.

In fact, he said, when he met with his international Council of Cardinals before leaving on the Cuba-Mexico trip, it was decided that the doctrinal congregation should have a new adjunct secretary to oversee the Vatican investigations of abuse allegations against priests.

Asked what should be done with a bishop who simply transfers an accused priest from one parish to another, Pope Francis said such a bishop is “a man without a conscience, and the best thing he can do is present his resignation. Is that clear?”

The sexual abuse of a child by a priest is “a monstrosity,” he said.

While Pope Francis was in Mexico, the BBC ran a story on previously unpublished letters from St. John Paul II to Anna-Teresa Tymieniecka, a philosopher. Many news reports on the letters raised doubts about the relationship being strictly a platonic friendship.

Pope Francis said the friendship was not a secret; “I already knew about this friendship between St John Paul II and this philosopher when I was in Buenos Aires.”

“A man who does not know how to have a relationship of friendship with a woman — I’m not talking about misogynists; those are sick — well, he’s a man who is missing something,” the Pope said.

In his own experience, he said, it is important to get a woman’s opinion when making decisions because “they look at things in a different way.”

Even for a priest or a pope, he said, “a friendship with a woman is not a sin, it’s a friendship. A romantic relationship with a woman who is not your wife — that is sin. Understand?

“But the pope is a man. The pope needs the input of women, too. And the pope, too, has a heart that can have a healthy, holy friendship with a woman. There are saint-friends — Francis and Clare, Teresa and John of the Cross — don’t be frightened,” he told reporters.

Pope Francis recognised that priests’ friendships with women are still suspect, which is a shame, he said. “We have not understood the good that a woman can do for the life of a priest and of the Church in the sense of counsel, help, healthy friendship.”

The Pope also was asked about an interview Archbishop Sviatoslav Shevchuk of Kiev-Halych, major archbishop of the Ukrainian Catholic Church, gave on Saturday in which he said Ukrainians felt “deep disappointment” over the joint declaration.

“When I read this, I was worried,” said the Pope, who explained that he has known and respected Archbishop Shevchuk for years.

The archbishop’s criticism seemed “a bit strange,” he said, but when people speak, their words must be read in the context of what they are living. The Ukrainians have the experience of Russian aggression toward the Ukrainian Catholic Church and Russian support for separatist fighting in Eastern Ukraine. That experience cannot be ignored, he said.

“You can understand how people in that situation feel this way,” the Pope said. The archbishop’s right to express his opinion must be respected, he said, “especially in this situation.”

In a statement in response to the Pope’s comments, Donald Trump said: “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 because this would not have happened. ISIS would have been eradicated unlike what is happening now with our all talk, no action politicians.

“The Mexican government and its leadership has made many disparaging remarks about me to the Pope, because they want to continue to rip off the United States, both on trade and at the border, and they understand I am totally wise to them. The Pope only heard one side of the story – he didn’t see the crime, the drug trafficking and the negative economic impact the current policies have on the United States. He doesn’t see how Mexican leadership is outsmarting President Obama and our leadership in every aspect of negotiation.

“For a religious leader to question a person’s faith is disgraceful. I am proud to be a Christian and as President I will not allow Christianity to be consistently attacked and weakened, unlike what is happening now, with our current President. No leader, especially a religious leader, should have the right to question another man’s religion or faith. They are using the Pope as a pawn and they should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant.”