7 You must labor, venerable brothers, that Catholic teaching about the sanctity, oneness, and perpetuity of matrimony takes firm root in souls. Remind the faithful frequently that the marriage of Christians is subject solely to ecclesiastical authority. Remind them also what the Church thinks and teaches concerning so called civil marriage and with what mind and heart Catholic people should obey such laws. Further remind them that even for the gravest of reasons it is not permitted to enter into marriage with Christians who are not Catholics; those who do so without the authority and indulgence of the Church sin before God and the Church. Since these issues are so vital, all who have a concern in this matter should most diligently see to it, as far as they can, that no one sins here for any reason. For in this especially, obedience to the Church is necessarily bound to the public interest. This is the reason why the beginnings and best principles of civil life depend in great part on domestic society, so that the peace and prosperity of the state result in large part from marriage. Nor can marriage succeed except under the care of God and the Church. Deprived of such care and entered upon contrary to the will of God, matrimony is reduced to the service of various passions, is deprived of necessary heavenly aids, and is despoiled of that common life which is of greatest concern to man, i.e., religion. Of necessity it produces bitter fruit, to the great harm of the family and of the state. For this reason We must commend those Catholic men who, when the legislative assembly of Hungary was asked two years ago whether it would consider the marriage of Christians with Jews valid, rejected the proposal unanimously and freely and succeeded in having the old marriage law retained. Their vote received the approval of the vast majority of people from all parts of Hungary, proving with admirable testimony that the people thought and felt as they did. May there be Like consent and similar constancy whenever the Catholic cause is in controversy, for then victory will be at hand. At least civil life will be more vigorous and fruitful when languor and sloth have been banished, for these are the means by which the enemies of the Christian name certainly wish to stupefy all Catholic virtue.
Source: Quod Multum (Vatican.va)