In his packed 5-day trip to Mexico, Pope Francis didn’t wait long to address one of the more controversial topics of his visit—immigration—calling it “the challenge of our age.”

In an address to a group of Mexican bishops, Francis expressed his appreciation for “everything you are doing” to confront this challenge, and urged them to team up with bishops from north of the border to offer ongoing pastoral care to those who leave their homeland.

“There are millions of sons and daughters of the Church who today live in the diaspora or who are in transit, journeying to the north in search of new opportunities,” Francis said. “Many of them have left behind their roots in order to brave the future, even in clandestine conditions which involve so many risks.”

“So many families are separated; and integration into a supposedly ‘promised land’ is not always as easy as some believe,” he said.

The Pope avoided the question of immigration policy, stressing rather the pastoral responsibility of priests and bishops to care for the faith of those who are far from home.

He invited the Mexican bishops not to forget members of their flock who have gone abroad, insisting rather that they need hearts “capable of following these men and women and reaching them beyond the borders.”

“Strengthen the communion with your brothers of the North American episcopate, so that the maternal presence of the Church can keep alive the roots of the faith of these men and women, as well as the motivation for their hope and the power of their charity,” he said.

Francis suggested that much of what the Church has to offer is its understanding of the universal brotherhood of all humanity under the fatherhood of God, asking the bishops to witness together with the U.S. bishops that “the Church is the custodian of a unifying vision of humanity and that she cannot consent to being reduced to a mere human ‘resource.’”

The Pope is expected to return to the topic of immigration, especially on the final day of his visit. Next Wednesday, Francis will visit the border town of Ciudad Juárez, where he will participate in several events, including the celebration of a “cross-border” Mass, attended by both Mexicans and Americans.

The Mass is scheduled to take place at El Punto, a large field near Benito Juarez Stadium and just a short distance from the U.S. border. Organizers say that tickets will be furnished to Catholic parishes on both sides of the border for those who want to attend the Mass. The venue holds approximately 220,000 people.

“During Mass, Pope Francis will undoubtedly call attention to many realities that are lived on both sides of our U.S.-Mexico border, particularly the plight of so many migrants and refugees fleeing violence and poverty in their home countries, in search of better lives for themselves and their children,” said El Paso Bishop Mark J. Seitz in a statement.

Last week, Presidential candidate Donald Trump criticized the pontiff’s stance on immigration, calling him a “very political person” who is being exploited by Mexico at America’s expense.

On Fox Business Network Thursday morning, Stu Varney reminded Donald Trump that the Pope is going “to stand at the border” with migrants, and asked for his take on the matter.

“I think that the Pope is a very political person,” Trump said, suggesting that he thinks the Pope is being used by Mexico to advance its interests against the United States.

“I think he doesn’t understand the problems our country has,” he continued. “I don’t think he understands the danger of the open border that we have with Mexico. And I think Mexico got him to do it because Mexico wants to keep the border just the way it is because they’re making a fortune and we’re losing.”

Follow Thomas D. Williams on Twitter @tdwilliamsrome