34 And this inviolable stability, although not in the same perfect measure in every case, belongs to every true marriage, for the word of the Lord: "What God hath joined together let no man put asunder," must of necessity include all true marriages without exception, since it was spoken of the marriage of our first parents, the prototype of every future marriage. Therefore although before Christ the sublimeness and the severity of the primeval law was so tempered that Moses permitted to the chosen people of God on account of the hardness of their hearts that a bill of divorce might be given in certain circumstances, nevertheless, Christ, by virtue of His supreme legislative power, recalled this concession of greater liberty and restored the primeval law in its integrity by those words which must never be forgotten, "What God hath joined together let no man put asunder." Wherefore, Our predecessor Pius VI of happy memory, writing to the Bishop of Agria, most wisely said: "Hence it is clear that marriage even in the state of nature, and certainly long before it was raised to the dignity of a sacrament, was divinely instituted in such a way that it should carry with it a perpetual and indissoluble bond which cannot therefore be dissolved by any civil law. Therefore although the sacramental element may be absent from a marriage as is the case among unbelievers, still in such a marriage, inasmuch as it is a true marriage there must remain and indeed there does remain that perpetual bond which by divine right is so bound up with matrimony from its first institution that it is not subject to any civil power. And so, whatever marriage is said to be contracted, either it is so contracted that it is really a true marriage, in which case it carries with it that enduring bond which by divine right is inherent in every true marriage; or it is thought to be contracted without that perpetual bond, and in that case there is no marriage, but an illicit union opposed of its very nature to the divine law, which therefore cannot be entered into or maintained."
Source: Casti Connubii (Vatican.va)