The Catholic Church teaches that marriage is, by God’s plan, an enduring and exclusive partnership between a man and a woman for the giving and receiving of love and the procreation and education of children.
The Church presumes that every marriage (Catholic, Protestant, Jewish, non-believer, etc.) is a true and valid union. Therefore, all previous marriages must be examined by the Tribunal before a person may be declared free to remarry in the Catholic Church.
A Church annulment is a declaration by the Catholic Church in a particular diocese that a specific union, presumably begun in good faith, and thought by all to be a marriage, was in fact an invalid union according to the Catholic Church’s most recent teachings of sacramental theology and Canon Law.
An annulment does not deny that a real relationship existed, but it is a statement by the Church that the relationship fell short of at least one of the elements considered essential for a binding marital union.