Here is the way the Catholic Church regards marriage.
It’s considered a sacrament, which the Church defines as "efficacious signs of grace, instituted by Christ and entrusted to the Church, by which divine life is dispensed to us.” That is, in the sacraments, God may be especially near to us. Moreover, marriage reflects the love of Christ for his Church (I would rather say for his people, the People of God).So it is VERY serious.
Catholics are expected to marry in a church and to make every effort to raise their children Catholic, and of course to be faithful to each other “until death do you part.” The Church recognizes only marriage between a man and a woman.
No blame is placed on a person who is divorced. But if someone wants to remarry, they have to get an annulment. To do so, they find an advocate (usually the parish priest) who petitions the diocesan Marriage Tribunal and provides a description of the case and a list of witnesses who know the couple and can testify about the marriage.
The procedure is carried out under Canon Law, the legal code which governs all aspects of the Church. If the Court agrees to take the case, the witnesses are contacted and asked to provide information and opinions about the marriage. The petitioner and the respondent do the same. A judge is assigned and the evaluation of the case begins. There may be more than one judge, and there is always a Defender of the Bond, who offers arguments why the marriage should not be annulled. There may be other officials, such as Auditors, who also consider the evidence and the law of the case and offer their opinions to the judge.
The judge considers the case in the light of the facts as they relate to Canon Law. There are a number of different causes for which the previous marriage may be considered invalid. These range from someone being kidnapped for marriage to one party being a priest. There are more common causes as well, such as mental disorder, fraud, simulation, and infidelity. The judgment is based on the way things were at the time of the marriage, not later on. So, for example, if someone carries on an affair after twenty years of marriage, that cannot be a cause unless it can be proved that the person intended that from the beginning.
When the judge reaches a decision, he (or she) issues a “sentence,” which decrees whether or not the annulment is granted. Most cases in he United States are adjudged in favor of the petitioner.
So that’s it. By the way, it isn’t necessary in a Catholic marriage that both partners be Catholic.