Divorce, in and of itself is not a sin...the Catholic could be the innocent partner. Divorce and remarriage, however, constitutes an adulterous relationship and is sinful.
Actually, divorce may be a mortal sin, or no sin.
A Catholic is required to consult with his pastor about divorce—a requirement of which virtually all Catholics are ignorant. I think this is the reason one hears of priests telling people WHO HAVE NOT REMARRIED that they must not receive Communion.
Amoris laetitia, btw, contains heresy. The next good Pope will repudiate it.