From Casti Connubii §5: … 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 it but by God, the Author of nature, and by Christ Our Lord by Whom nature was redeemed, and hence these laws cannot be subject to any human decrees or to any contrary pact even of the spouses themselves.

“Remarriage” is a word that is being used very imprecisely in an important public discussion about reception of Holy Communion. In reality, a remarriage is a marriage that is contracted by someone whose previous marriage ended with the death of the other spouse. This is the only sense in which the word “remarriage” has any meaning, because the death of the spouse is the only way a marriage can end.