The Catholic Church has a very genuine and pastoral concern for its members and their growth in holiness. Essential to this is their sacramental life in the Church. Consequently, the Church wants to be involved in their preparation and celebration of the Sacrament of Holy Matrimony.
The Church requires that a Catholic party must be married before a duly-authorized priest or deacon in the presence of two witnesses. This is called the canonical form of a marriage. If a Catholic is married before a justice of the peace, a civil judge, or a minister of another Christian church, then the marriage is not considered valid in the eyes of the Church. This is called a lack of form. The bishop can grant a dispensation from the requirement of canonical form, but it must be done before the wedding.
Note that the priest or deacon must also be “duly-authorized.” This means that the minister must have been given appropriate delegation (loosely, “permission”) to serve as a witness for a marriage. Delegation is usually given by the pastor of the parish where the parties marry. A visiting celebrant must obtain proper delegation, or the marriage is invalid. If a priest or deacon did not have the appropriate delegation prior to witnessing a wedding, then it is considered a defect of form, which invalidates the marriage (and yes, this does happen!).
Click the following link to download the Petition for Lack of Canonical Form