The Catholic Church believes that a valid, sacramental marriage (between a baptized man and a baptized woman) cannot be dissolved except through death. The biblical basis for this is Matthew 19:6: “So they are no longer two, but one flesh. Therefore, what God has joined together, no human being must separate.”
What appears to be a valid, sacramental marriage, however, may not be that. There are many reasons why a union between a man and a woman might be declared null. For example, one person’s “I do” could mean “as long as I like how this relationship is going.” If such an intention could be proven by testimony from firsthand witnesses, then the marriage might be declared null because one partner was not making a permanent commitment.