On Attending a Marriage Outside the Church

From the Dear Father column in the St Louis Review, a cogent and thoughtful answer is given by Fr. Matthew Mitas to a person who asks:
"Is it permissible for a Catholic to be a witness at the wedding of another Catholic who is marrying outside the Church?".

It depends on what you mean by "marrying outside the Church." In a case like this, you want to make sure whether this marriage will be valid in the eyes of the Church.

It is possible that a marriage taking place in a Protestant church has been given a dispensation from the bishop. Also, if the Catholic in question has formally and publicly denounced his Catholic faith (for example, by joining another religion) then he is no longer a subject of Catholic Church law and thus not required to be married in the presence of a priest.

In either case, a Catholic could serve as a witness in good conscience. In most cases like the one you describe, however, the marriage truly is invalid, and no Catholic may serve as a witness to it.

At the heart of this lies scandal. Jesus’ last words to us were a command that we teach, make disciples of and baptize all nations. He expects every one of us to promote the truth and virtue of the Gospel. Not only that, but he saved his harshest condemnation for those who give scandal (see Matthew 18:5). Teaching along the same lines, St. Paul makes it clear that even when a behavior is otherwise innocent, if it causes someone to be scandalized, it must not be done (1 Corinthian 8:1-13).

An important distinction must be made. There’s a difference between behavior that shocks and that which truly scandalizes. Shocking though it may be, it scandalizes only when it causes someone else to lower his own standards, imitate the bad behavior or refrain from any good or virtuous act he might otherwise have done. It’s conceivable that one man’s shocking behavior may actually jolt another into reaffirming his own sense of righteousness. When scandal occurs, it is a gravely serious matter.

I bring this up to make a point, that we Catholics have already been scandalized with regard to marriage. We’ve lost our sense of marriage as being the sacred thing that Christ established, only to descend into our society’s attitudes toward marriage. I’m referring to the silly notions that love is a feeling and not a commitment (and so people fall out of love as quickly as they fall in it), that a wedding is something "romantic" and not holy (and so they seek to be married in a place merely because it’s pretty) and the selfish belief that a person gets married for his own happiness and not as a response to a vocation from God. As a consequence, divorce is seen by many as an easy solution to a spouse’s not feeling fulfilled.

Years ago, entertainer Jackie Gleason underwent rehab for alcoholism. When he came out, he was quoted as saying that all the while he was boozing, nobody ever told him it was wrong. It seemed preposterous; anybody should know that that’s not the way to live. He was probably right; I suspect he was surrounded by people who lacked the courage to confront him.

It’s the duty of Catholics, especially in this day of scandal, to stand tall for the faith. It’s high time that when a Catholic is asked to compromise his faith to take part in a marriage outside the Church, he should remind his brother of his obligations before God for the sake of his soul. This is what Christ commanded (Matthew 18:15-17). God forbid it should lead to scandal. (my emphasis)

Source.