Nationalism, in its historic and simplest sense, is patriotism applied to one’s nationality. As such it is a common cultural phenomenon, and one compatible with Catholic tradition and precept. For patriotism is a love of one’s country, a prime expression of that sense of loyalty which holds men together in groups and without which men could not be the gregarious creatures that by nature they are.
Men have always lived in groups. Apparently, it is a part of God’s plan that they should. And one of the things which have enabled them to live in groups has been the loyalty—the patriotism—which God has implanted in their very nature. This loyalty—this patriotism—this “love of country”—involves a triple affection. It embraces an affection for familiar places, an affection for familiar persons, and an affection for familiar ideas. One’s “country” connotes all three of these: the land itself, the persons on it, and the traditions associated with it. One’s “native land”—the terra patria, la patrie, das Vaterland—is an extension of hearth and home. It is the soil that has given life to one’s forefathers and holds their tombs, and which in turn nurtures one’s children and grandchildren.
It is a link between generations, between families and friends, between common experience of the past and that of the present and future. It is the earthly means, at once familiar and sacred, of establishing and maintaining a community or group life.
Loyalty to “country,” or patriotism in the basic sense, is one of those fine traits of man which prompt him to rise out of himself, to do things for his fellows, to have a regard for order and tradition and ideals. Patriotism is not merely passive loyalty to country. It is active love of country. But true patriotism, like any true love, is cherished in humility, not in pride. It is, of course, primarily emotional and impulsive, rather than deliberate and reasoned, though like any human emotion, such as hunger or passion, it is liable to grave abuse if it is not guided by reason and disciplined by experience.
The right use of patriotism is a precept of the natural law and a Christian duty. Such has been the invariable teaching of the Catholic Church and the Roman Pontiffs. In the words of Pope Leo XIII: “The natural law enjoins us to love devotedly and to defend the country in which we had birth and in which we were reared, so that every good citizen hesitates not to face death for his native land.” And Pope Pius XI has added that patriotism is “the stimulus of so many virtues and of so many noble acts of heroism when kept within the bounds of the law of Christ.”
“When kept within the bounds of the law of Christ.” This, too, has invariably been a reservation which the Catholic Church and the Roman Pontiffs have made to their inculcation of patriotism. Duties to our compatriots must not blind us, the Church constantly proclaims, to our paramount duties to God, or to the duties of justice and charity which we owe to mankind at large. Christ Himself instructed us at the beginning “to render unto Caesar the things that are Caesar’s, and to God the things that are God’s.” Pope Pius XI reminds us in the present age that “love of country becomes merely an occasion for, and an added incentive to, grave injustice, when true love of country is debased to the condition of an extreme nationalism, when we forget that all men are our brothers and members of the same great human family, that other nations have an equal right with us both to life and to prosperity, that it is never lawful nor even wise to dissociate morality from the affairs of practical life, that, in the last analysis, it is ‘justice which exalteth a nation, but sin maketh nations miserable.’” – from “Patriotism, Nationalism and the Brotherhood of Man”  (1937)