79 We have so far, Venerable Brethren, shown the excellency of the first two blessings of Christian wedlock which the modern subverters of society are attacking. And now considering that the third blessing, which is that of the sacrament, far surpasses the other two, we should not be surprised to find that this, because of its outstanding excellence, is much more sharply attacked by the same people. They put forward in the first place that matrimony belongs entirely to the profane and purely civil sphere, that it is not to be committed to the religious society, the Church of Christ, but to civil society alone. They then add that the marriage contract is to be freed from any indissoluble bond, and that separation and divorce are not only to be tolerated but sanctioned by the law; from which it follows finally that, robbed of all its holiness, matrimony should be enumerated amongst the secular and civil institutions. The first point is contained in their contention that the civil act itself should stand for the marriage contract (civil matrimony, as it is called), while the religious act is to be considered a mere addition, or at most a concession to a too superstitious people. Moreover they want it to be no cause for reproach that marriages be contracted by Catholics with non-Catholics without any reference to religion or recourse to the ecclesiastical authorities. The second point which is but a consequence of the first is to be found in their excuse for complete divorce and in their praise and encouragement of those civil laws which favor the loosening of the bond itself. As the salient features of the religious character of all marriage and particularly of the sacramental marriage of Christians have been treated at length and supported by weighty arguments in the encyclical letters of Leo XIII, letters which We have frequently recalled to mind and expressly made our own, We refer you to them, repeating here only a few points.
Source: Casti Connubii (Vatican.va)