To lay down a precise rule for every case is a difficulty beyond human capacity; but some general rules of guidance may be given; their application must be left to the prudence of those who are individually concerned according to their circumstances and special obligations.
It will be well first to distinguish, in a general way, three possible relations between a Catholic and Liberalism or rather between a Catholic and Liberals:
1. Necessary relations;
2. Useful relations;
3. Relations of pure affection or pleasure.
Necessary relations are imposed upon every one by his ration in life and his particular position; they cannot be avoided. Such are the family relations, the relations of inferior and superior, etc.
It is evident that a son who has the misfortune to have a Liberal father cannot on this account abandon him, nor the wife the husband, the brother the sister, or the parent the child, except in the case where their Liberalism exacts from any of their respective inferiors acts essentially opposed to religion so as to conduce to a formal apostacy.
But it will not suffice, on the part of the Catholic, for the taking of such a step that mere restraint is put upon his liberty in the performance of the precepts of the Church. For we must remember that the Church places no obligation in such matters on a person who could only perform them under grave inconvenience (sub gravi incommode).
The Catholic unfortunate enough to be so placed must bear with Christian patience his painful situation and surround himself, as far as lies in his power, with every precaution to avoid the contagion of bad example in word or deed. Prayer should be his chief recourse, prayer for himself and the victims of error. He should avoid as far as possible, all conversations on this topic, but when he finds that a controversy is thrust upon him, let him accept it in the full confidence of the truth and armed with effective weapons of defense and offense. A prudent spiritual director should be consulted in the selection of his arsenal. As an antidote to much associate with Liberals, let him frequent the company of other persons of science and authority who are in the constant possession of sound doctrine. Obedience to a superior in all that is not directly or indirectly against faith and morals is his bounded duty, but it is equally his duty to refuse obedience to anything directly or indirectly in opposition to the integrity of his faith. Courage he an draw only from supernatural sources; God who sees the struggle will not refuse all the assistance needed.
There are other relations which we have with Liberals, not absolutely, but morally indispensable, and without which social life, which consists in a mutual exchange of services, is impossible. Such are the relations of commerce, trade, labor, the professions, etc. But that struck subjection, which holds under the necessary relations of which we have just been speaking, does not exist here, and in consequence one can exercise more independence. The fundamental rule in these cases is not to enter into unnecessary intercourse; what the gearing of the social machine demands, and no more, is sufficient. If you are a merchant buy and sell with Liberals in accordance with the needs of your business; more than this avoid; if you are domestic limit your intercourse to the necessities of your service; if you are a laborer, to giving and receiving what is due on either part. Guided by these rules one could live without injury by his faith admits a population of Jews. At the same time, it should never be forgotten that any manifestation of weakness or compromise is never needed. Even Liberals cannot refuse respect to the man who stands firmly and unflinchingly on his conviction, and when the faith is in question, despicable in all men's eyes does he become who would sell his birthright for a mess of pottage.
Relations of pure friendship, pleasure or affection, which we enter into as mere matters of taste or inclination, should be eschewed and, if once contracted, ought to be voluntarily broken off. Such relations are certain danger to our faith. Our Lord says that he who loves danger shall perish in it. It is difficult to sever such connections? What if it is; we must burst the bonds that place us in peril. Reflect for a moment. If your Liberal companion, with whom you are constantly associating, were subject to some contagious disease, would you then court him? If your relations with him compromised your reputation, would you continue them? If he were to asperse your family would you cling to him still? Well, the honor of God and your own spiritual safety is at stake in this matter; what human prudence would counsel you to do for your worldly interest and human honor, surely that much at least your spiritual interests require from you. There is but one condition upon which intimacy with a Liberal is justifiable at all, and that is, for the purpose of converting him; for this two dispositions are necessary: your Liberal friend's willingness and your capacity to lead him to the light. Even here danger is not lacking. One must be very sure of his ground before he attempts the task.
Above all have a horror of heresy, and Liberalism today is the most malignant of all heresies. Its face is set against religious faith absolutely. The first thing to do in an infected country is to isolate oneself, and if this is not possible take all sanitary precautions against the deadly germ. Spiritual health is always endangered whenever we come in contact with Liberalism, and infection is almost certain if we neglect those precautions which prudence suggests.
Source: What is Liberalism, Imprimatur 1899