At the website of the U.S. Conference of Catholic Bishops, we read:
Why is the Catholic Church involved in the immigration issue? There are several reasons the Catholic Church is involved in the immigration debate. The Old and New Testaments, as well as the encyclicals of the Popes, form the basis for the Church's position. In Gospel of Matthew, Jesus calls upon us to "welcome the stranger,for what you do to the least of my brethren, you do unto me. " (Mt. 25-35, 40).
There is a deep mistake being made here, and we should try to understand what it is. The mistake is to confuse the private and public spheres and the different moralities pertaining to each.
The problem of confusing private and public morality is well understood by Hannah Arendt ("Truth and Politics" in Between Past and Future, Penguin, 1968, p. 245):
The disastrous consequences for any community that began in all earnest to follow ethical precepts derived from man in the singular -- be they Socratic or Platonic or Christian -- have been frequently pointed out. Long before Machiavelli recommended protecting the political realm against the undiluted principles of the Christian faith (those who refuse to resist evil permit the wicked "to do as much evil as they please"), Aristotle warned against giving philosophers any say in political matters. (Men who for professional reasons must be so unconcerned with "what is good for themselves" cannot very well be trusted with what is good for others, and least of all with the "common good," the down-to-earth interests of the community.) [Arendt cites Nicomachean Ethics, Book VI, and in particular 1140b9 and 1141b4.]
There is a tension between man qua philosopher/Christian and man qua citizen. As a philosopher/Christian, I am concerned with my soul, with its integrity, purity, salvation. I take very seriously indeed the Socratic "Better to suffer wrong than to do it" and the Christian "Resist not the evildoer." But as a citizen I must be concerned not only with my own well-being but also with the public welfare. This is true a fortiori of public officials and people in a position to influence public opinion, people like Catholic bishops. So, as Arendt points out, the Socratic and Christian admonitions are not applicable in the public sphere.
A Catholic bishop, therefore, who is pro illegal immigration on the strength of the "welcome the stranger" passage demonstrates a failure to understand the simple point that Arendt underscores.
What is applicable to me in the singular, as this existing individual concerned with the welfare of his immortal soul over that of his perishable body, is not applicable to me as citizen. As a citizen, I cannot "welcome the stranger" who violates the laws of my country, a stranger who may be a terrorist or a drug-smuggler or a human-trafficker or a carrier of a deadly disease or a person who has no respect for the traditions of the country he invades; I cannot aid and abet his law-breaking. I must be concerned with public order and the very conditions that make the philosophical and Christian life possible in the first place. If I were to aid and abet the stranger's lawbreaking, I would not be "rendering unto Caesar the things that are Caesar's."
Indeed, the Caesar verse provides a scriptural basis for Church-State separation and indirectly exposes the fallacy of the Catholic bishops who cannot comprehend the simple distinctions I have tried to set forth.