6 Moreover, if we would judge aright, the supernatural love for the Church and the natural love of our own country proceed from the same eternal principle, since God Himself is their Author and originating Cause. Consequently, it follows that between the duties they respectively enjoin, neither can come into collision with the other. We can, certainly, and should love ourselves, bear ourselves kindly toward our fellow men, nourish affection for the State and the governing powers; but at the same time we can and must cherish toward the Church a feeling of filial piety, and love God with the deepest love of which we are capable. The order of precedence of these duties is, however, at times, either under stress of public calamities, or through the perverse will of men, inverted. For, instances occur where the State seems to require from men as subjects one thing, and religion, from men as Christians, quite another; and this in reality without any other ground, than that the rulers of the State either hold the sacred power of the Church of no account, or endeavor to subject it to their own will. Hence arises a conflict, and an occasion, through such conflict, of virtue being put to the proof. The two powers are confronted and urge their behests in a contrary sense; to obey both is wholly impossible. No man can serve two masters,(3) for to please the one amounts to contemning the other.
Source: Sapientiae Christianae (Vatican.va)