Pope Francis offered a subtle rebuke to President Trump this week, in a letter that urged Christians not to close the door on anyone, and said " no people is criminal and no religion is terrorist."

The pope's letter was read by a cardinal representing the Vatican on Thursday, and while it didn't mention Trump by name, it was written after Trump's controversial executive order aimed at temporarily banning immigrants from seven Muslim-majority countries.

"As Christians and all people of good will, it is for us to live and act at this moment. It is a grave responsibility, since certain present realities, unless effectively dealt with, are capable of setting off processes of dehumanization which would then be hard to reverse," the pope wrote in the letter read by Cardinal Peter Turkson, head of the Vatican's department for Integral Human Development.

"The grave danger is to disown our neighbors," the pope wrote. "When we do so, we deny their humanity and our own humanity without realizing it; we deny ourselves, and we deny the most important Commandments of Jesus."

In person, the pope challenged countries weighing whether to accept migrants fleeing conflict in the Middle East not to view these people as dangerous. Instead, he said they should be seen as an opportunity.

In a discussion with students at a Rome university on Friday, the pope was asked by a 31-year-old married mother and Syrian refugee what he thought about the threat posed by Syrian and Iraq to Christians in Europe.

"Migrations are not a danger, but a challenge to grow," was his response, according to Vatican Radio.