87 Opposed to all these reckless opinions, Venerable Brethren, stands the unalterable law of God, fully confirmed by Christ, a law that can never be deprived of its force by the decrees of men, the ideas of a people or the will of any legislator: "What God hath joined together, let no man put asunder." And if any man, acting contrary to this law, shall have put asunder, his action is null and void, and the consequence remains, as Christ Himself has explicitly confirmed: "Everyone that putteth away his wife and marrieth another, committeth adultery: and he that marrieth her that is put away from her husband committeth adultery." Moreover, these words refer to every kind of marriage, even that which is natural and legitimate only; for, as has already been observed, that indissolubility by which the loosening of the bond is once and for all removed from the whim of the parties and from every secular power, is a property of every true marriage.
Source: Casti Connubii (Vatican.va)