39 Let special care be taken that the people be well instructed in the precepts of Christian wisdom, so that they may always remember that marriage was not instituted by the will of man, but, from the very beginning, by the authority and command of God; that it does not admit of plurality of wives or husbands; that Christ, the Author of the New Covenant, raised it from a rite of nature to be a sacrament, and gave to His Church legislative and judicial power with regard to the bond of union. On this point the very greatest care must be taken to instruct them, lest their minds should be led into error by the unsound conclusions of adversaries who desire that the Church should be deprived of that power.
Source: Arcanum Divinae (Vatican.va)