In the Catholic Church, a declaration of marriage nullity is a judgement reached by an ecclesiastical tribunal that a given marriage was invalidly contracted at the moment of consent. The judgment is given only after a painstaking “judicial process”, that is, as the result of an investigation carried out by an ecclesiastical judge. The investigation is carried out in accordance with the Church’s own legal procedures as contained in the 1983 Code of Canon Law. This Code includes laws about preparing for marriage, getting married, living married life and deciding whether or not a marriage brought before a tribunal is null.
A declaration of nullity is not the breaking of an existing marriage bond, but rather the affirmation that the bond never actually came into existence at all due to the absence of one or more of the essential requirements for valid matrimonial consent. A declarion of nullity does not therefore create a new reality but turns the spotlight back onto an old one. It declares the bond to be, and therefore always to have been, invalid; it does not actually invalidate the bond, as if untying a ribbon bow, in the way that divorce claims to do. Rather, it declares that, despite appearances, the bow was never actually tied.
There are three main categories of deficiencies which could prevent a marriage bond from being valid at the exchange of consent between the betrothed. They are explained in more detail elsewhere on this website, but here is a brief explanation of each category:
The first category is that of the "invalidating impediment” which may, or may not, have been known at the time of the marriage. For example, a couple only discovers later in their marriage that they are actually brother and sister. This close blood relationship impedes the existence of a valid marriage bond.
The second category is that of “deficient consent.” For example, a woman only discovers after some time that her husband had deliberately deceived her about a vasectomy he had undergone years before they met. The deceit was perpetrated in order not to lose her marriage consent. Deception of this kind means that the deceived spouse did not really know the full truth of the person whom she was marrying in a matter which is so significant for marital harmony, i.e. the ordering of the marriage towards the gift of children.
The third category regards the “canonical form” of the celebration of marriage. For example, if a Catholic marries in a non-Catholic Christian Church without seeking a dispensation to do so, the marriage would be null by “defect of form.”