Let it be repeated as an immutable and inviolable fundamental doctrine that matrimony was not instituted or restored by man but by God. Not by man were the laws made to strengthen and confirm and elevate marriage, but by God, the Author of nature, and by Christ Our Lord by Whom nature was redeemed; and hence these [marriage] laws cannot be subject to any human decrees or to any contrary pact, even of the spouses themselves.
Yet, although matrimony is of its very nature of divine institution, the human will, too, enters into it and performs a most noble part. For each individual marriage, inasmuch as it is a conjugal union of a particular man and woman, arises only from the free consent of each of the spouses; and this free act of the will, by which each party hands over and accepts those rights proper to the state of marriage, is so necessary to constitute true marriage, that it cannot be supplied by any human power…
Therefore, the sacred partnership of true marriage is constituted both by the will of God and the will of man. From God comes the very institution of marriage, the ends [purpose] for which it was instituted, the laws that govern it, the blessings that flow from it. Man, through generous surrender of his own person made to another for the whole span of life, becomes, with the help and cooperation of God, the author of each particular marriage, with the duties and blessings annexed to it from divine institutions.
Pope Pius XI December 31, 1930