9 But what was decreed and constituted in respect to marriage by the authority of God has been more fully and more clearly handed down to us, by tradition and the written Word, through the Apostles, those heralds of the laws of God. To the Apostles, indeed, as our masters, are to be referred the doctrines which "our holy Fathers, the Councils, and the Tradition of the Universal Church have always taught,"(9) namely, that Christ our Lord raised marriage to the dignity of a sacrament; that to husband and wife, guarded and strengthened by the heavenly grace which His merits gained for them, He gave power to attain holiness in the married state; and that, in a wondrous way, making marriage an example of the mystical union between Himself and His Church, He not only perfected that love which is according to nature,(10) but also made the naturally indivisible union of one man with one woman far more perfect through the bond of heavenly love. Paul says to the Ephesians: "Husbands, love your wives, as Christ also loved the Church, and delivered Himself up for it, that He might sanctify it. . . So also ought men to love their wives as their own bodies. . . For no man ever hated his own flesh, but nourisheth and cherisheth it, as also Christ doth the Church; because we are members of His body, of His flesh, and of His bones. For this cause shall a man leave his father and mother, and shall cleave to his wife, and they shall be two in one flesh. This is a great sacrament; but I speak in Christ and in the Church."(11) In like manner from the teaching of the Apostles we learn that the unity of marriage and its perpetual indissolubility, the indispensable conditions of its very origin, must, according to the command of Christ, be holy and inviolable without exception. Paul says again: "To them that are married, not I, but the Lord commandeth that the wife depart not from her husband; and if she depart, that she remain unmarried or be reconciled to her husband."(12) And again: "A woman is bound by the law as long as her husband liveth; but if her husband die, she is at liberty."(13) It is for these reasons that marriage is "a great sacrament";(14) "honorable in all,"(15) holy, pure, and to be reverenced as a type and symbol of most high mysteries.
Source: Arcanum Divinae (Vatican.va)