Amazing how many Catholics do not know what the Church teaches about marriage. I make sure every CCD class I teach learns these things. The Church has held this teaching since Jesus was on earth.
A good high level of the Church teaching on marriage is below (full text can be read at this link):
When Jesus came, he elevated matrimony to the same status it had originally possessed between Adam and Eve—the status of a sacrament. Thus, any valid marriage between two baptized people is a sacramental marriage and, once consummated, cannot be dissolved. Jesus, therefore, taught that if anyone so married divorces and remarries, that person is living in perpetual adultery, a state of mortal sin.
He said, “Every one who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery” (Luke 16:18; cf. Mark 10:11–12).
Paul was equally insistent on this fact, declaring, “Thus a married woman is bound by law to her husband as long as he lives. . . . Accordingly, she will be called an adulteress if she lives with another man while her husband is alive” (Rom. 7:2–3).
This applied, of course, only to sacramental marriages—those between baptized people. For marriages involving an unbaptized party, a different rule applied (1 Cor. 7:12–15).
In the midst of the Greco-Roman culture, which allowed for easy divorce and remarriage, the early Church Fathers proclaimed Christ’s teaching on the indissolubility of marriage—just as the Catholic Church does today in our modern, secular, easy-divorce culture (cf. Catechism of the Catholic Church 1614–1615). Other denominations have modified their teachings to accommodate the pro-divorce ethos that dominates modern culture, but the Catholic Church preserves the teaching of Jesus and the early Christians.
While their ex-spouses are alive, the only time that a baptized couple can remarry after divorce is when a valid sacramental marriage never existed in the first place. For example, for a marriage to be contracted, the two parties must exchange valid matrimonial consent. If they do not, the marriage is null. If the competent authority (a diocesan marriage tribunal) establishes this fact, a decree of nullity (commonly called an annulment) can be granted, and the parties are free to remarry (CCC 1629). In this case there is no divorce followed by remarriage in God’s eyes because there was no marriage before God in the first place, merely a marriage in the eyes of men.
If, however, the parties are genuinely and sacramentally married, then, while in some cases there may be good reasons for them to live apart and even to obtain a legal separation, in God’s eyes they are not free to remarry (CCC 1649).
This is not a commandment of men, but one that comes directly from Jesus Christ. As Paul said, “To the married I give charge,* not I but the Lord*, that the wife should not separate from her husband (but if she does, let her remain single or else be reconciled to her husband)—and that the husband should not divorce his wife” (1 Cor. 7:10-11).
Fortunately, God will ensure that the sacramentally married have the grace necessary to live out their marriage vows and either stay married or live continently. The sacrament of matrimony itself gives this grace. Whenever we face a trial, God ensures that we will have the grace we need. As Paul elsewhere says, “No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your strength, but with the temptation will also provide the way of escape, that you may be able to endure it” (1 Cor. 10:13).
As the following quotations from the early Church Fathers illustrate, they also recognized the seriousness of Christ’s teaching regarding the indissolubility of marriage.
“What then shall the husband do, if the wife continue in this disposition [adultery]? Let him divorce her, and let the husband remain single. But if he divorce his wife and marry another, he too commits adultery” (*The Shepherd *4:1:6 [A.D. 80]).
“In regard to chastity, [Jesus] has this to say: ‘If anyone look with lust at a woman, he has already before God committed adultery in his heart.’ And, ‘Whoever marries a woman who has been divorced from another husband, commits adultery.’ According to our Teacher, just as they are sinners who contract a second marriage, even though it be in accord with human law, so also are they sinners who look with lustful desire at a woman. He repudiates not only one who actually commits adultery, but even one who wishes to do so; for not only our actions are manifest to God, but even our thoughts” (*First Apology *15 [A.D. 151]).
Clement of Alexandria
“That Scripture counsels marriage, however, and never allows any release from the union, is expressly contained in the law: ‘You shall not divorce a wife, except for reason of immorality.’ And it regards as adultery the marriage of a spouse, while the one from whom a separation was made is still alive. ‘Whoever takes a divorced woman as wife commits adultery,’ it says; for ‘if anyone divorce his wife, he debauches her’; that is, he compels her to commit adultery. And not only does he that divorces her become the cause of this, but also he that takes the woman and gives her the opportunity of sinning; for if he did not take her, she would return to her husband” (*Miscellanies *2:23:145:3 [A.D. 208]).