2 We have treated this matter often in other letters, especially in Our apostolic letter of 10 February 1880. In it We emphasized the nature of Christian marriage, its strength, the care the Church has devoted to protecting its honor and its rights, and the role of civil authority in its regard. As it is evident that Christ, the son of God, redeemer and restorer of human nature, raised Christian marriage to the dignity of a sacrament, every Christian marriage is this sacrament. The matter of the contract can be separated in some sense from the nature of the sacrament. This means that while the civil authority retains in full its right to regulate the so-called civil effects, the marriage itself is subject to the authority of the Church. In addition, it is certain that Jesus, the redeemer of every race, abolished the custom of repudiation, strengthened marriage with holy power, and reinstated the law of permanence just as it was established by the will of God from the beginning. It follows then that the marriage of Christians when fully accomplished is holy, indivisible, and perfect. It cannot be dissolved for any reason other than the death of either spouse according to the holy words: "What God has joined, let no man divide." In so doing, Christ certainly intended to confer many benefits on the human race, for this institution most effectively preserves or restores morality, fosters the love of one spouse for the other, confirms families with divine strength, renews the education and protection of offspring, maintains the dignity of woman, and finally establishes the honor and the prosperity of familial and civil association in the most beneficial and excellent way.
Source: Dum Multa (Vatican.va)