Immigration and the care of migrants should be put on the front burner and not considered a second-tier issue, Pope Francis has insisted in a new teaching letter released Monday morning in the Vatican.

In his 44-page apostolic exhortation bearing the Latin title Gaudete et Exsultate (“Rejoice and Be Glad”), the pope said that Christians have no right to treat the plight of migrants as a minor moral issue because Jesus commanded his followers to welcome the stranger.

“We often hear it said that, with respect to relativism and the flaws of our present world, the situation of migrants, for example, is a lesser issue,” Francis wrote. “Some Catholics consider it a secondary issue compared to the ‘grave’ bioethical questions.”

“That a politician looking for votes might say such a thing is understandable, but not a Christian, for whom the only proper attitude is to stand in the shoes of those brothers and sisters of ours who risk their lives to offer a future to their children,” he continued.

“Can we not realize that this is exactly what Jesus demands of us, when he tells us that in welcoming the stranger we welcome him?” he asked.

The pontiff defended his continual insistence on the issue of migration by saying it was not just his pet cause but a central teaching of the bible.

“This is not a notion invented by some Pope, or a momentary fad. In today’s world too, we are called to follow the path of spiritual wisdom proposed by the prophet Isaiah to show what is pleasing to God,” he said.

Quoting from the Old Testament books of Exodus and Leviticus, Francis said that oppressing strangers is contrary to God’s will.

In his letter, the pope does not descend to the level of practical public policy concerning immigration, and elsewhere in the text criticizes groups of Christians who “give excessive importance to certain rules, customs or ways of acting,” as if only one viewpoint were acceptable. In this regard he allows for a plurality of possible solutions under the virtue of prudence.

Welcoming the stranger is not “a problem for politicians to sort out” but the task of every Christian in day to day life, he suggests.

In his insightful exploration of the topic of immigration in the Old Testament, the great medieval scholar Saint Thomas Aquinas noted that every nation has the right to determine who can be allowed to migrate to it and to establish immigration policies accordingly.

Aquinas analyzed how the ancient Israelites actually applied in practice God’s commandment to welcome the stranger, noting that the Jewish people of Old Testament times did not admit visitors from all nations equally, since those peoples closer to them were more quickly integrated into the population than those who were not as close.

People emigrating from certain nations were not admitted into Israel at all due to their hostility toward the Jewish people and culture.

Citizens of nations “with whom their relations had been hostile,” such as the Ammonites and Moabites, “were never to be admitted to citizenship,” he observed.

“The Amalekites, who were yet more hostile to them, and had no fellowship of kindred with them, were to be held as foes in perpetuity,” Aquinas added.

In his nuanced commentary, Aquinas also distinguished among three types of “strangers” in the Israel of the Old Testament.

First were “the foreigners who passed through their land as travelers,” much like modern day visitors with a travel visa.

Second were those who “came to dwell in their land as newcomers,” seemingly corresponding to resident aliens, living in the land but without the full benefits of citizenship.

A third case involved those foreigners who wished “to be admitted entirely to their fellowship and mode of worship.” Even here, dealing with those who wished to integrate fully into the life and worship of Israel required a certain order, Aquinas observed. “For they were not at once admitted to citizenship: just as it was law with some nations that no one was deemed a citizen except after two or three generations.”

“The reason for this was that if foreigners were allowed to meddle with the affairs of a nation as soon as they settled down in its midst,” Aquinas argued, “many dangers might occur, since the foreigners not yet having the common good firmly at heart might attempt something hurtful to the people.”

In other words, Aquinas taught that total integration of immigrants into the life, language, customs and culture (including worship, in this case) was necessary for full citizenship.

In his analysis of the issue of immigration in the Old Testament, Aquinas avoided the common error of focusing solely on the rights of immigrants without taking into account the common good and safety of the host nation, in this case Israel itself and its citizens.

One thing that was absolutely clear for Aquinas was that the host nation itself had the right and the duty to legislate immigration in such a way that it was of benefit not only to the strangers seeking admittance, but to the receiving nation as well.

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