Three factors are to be kept in mind regarding a valid sacrament of marriage. The law states that marriage is brought about (c. 1057) through: (1) the consent of the parties (the bride and groom), (2) legitimately manifested, (3) by those qualified according to the law (again, the bride and the groom).
Once the bride and groom exchange their vows, the marriage (c. 1055) that takes place is legally presumed (c. 1060) to be a valid sacrament. Yet if enough information is brought forward to indicate otherwise, the presumption of validity will yield to a declaration of nullity.
What kind of information is needed in a tribunal investigation regarding nullity? Information is gathered in regard to the consent of the parties, the manner in which the consent was manifested and the qualifications of the ministers to place an act of consent.
The consent of the parties
The most common nullity proceeding examines the first criteria, the consent of the two parties as it was exchanged on the wedding day. This is only logical as it was that consent which brought about the presumption of validity. This type of tribunal proceeding is commonly (though inappropriately) referred to as an annulment. As previously stated the better term is declaration of nullity.
Remember, marriage is brought about through the consent of the parties. However, if it can be proven that the consent was defective, on the part of one or both of the parties, then marriage was NOT brought about.
The bride and groom are the ministers of marriage
It is crucial to keep in mind that the bride and the groom are the ministers of the sacrament of marriage. The man gives the sacrament to the woman, the woman gives the sacrament to the man. As the Catechism of the Catholic Church states: "In the Latin Church, it is ordinarily understood that the spouses, as ministers of God's grace, mutually confer upon each other the Sacrament of Matrimony by expressing their consent before the Church (CCC, 1623)."
In spite of this theological tenet, more often than not, the priest or deacon at a wedding is considered by many to be the minister of the sacrament. Many call him the minister. Even his liturgical gesture of extending his arm in blessing over the couple can look as if he is conferring the sacrament. However, he is not.
The priest or deacon is an "official" witness of the faith community. Along with the best man and maid of honor, he is one of the three individuals who witness what the two ministers are doing. Our theology teaches that it is the consent of the two ministers which brings about the sacrament of marriage.
Consent is an "act of the will." Now that's a nice philosophical concept and hard to get your fingers on. Yet the parties manifest what their wills desire when they say "I do" or repeat the marriage vow.
The ministers' consent (cc. 1095-1107) is like a diamond, insofar as it is multi-faceted. For instance, it is a free act of the will, not a forced act. There is an understanding of what marriage is all about with its essential elements of permanence, fidelity, and conjugal love (which is open to children). So it is presumed that the ministers marry according to the mindset of the church.
There is good discretionary judgment on the part of both parties as they consent to marriage. In other words, there are no red flags raised saying to the individuals, "don't get married at this point in your life"; "don't marry this particular person"; "don't marry as the relationship is seriously troubled"; etc.
It is presumed that neither minister suffers from a psychological anomaly. For example, they are not schizophrenic, paranoid, or suffer from multiple personalities. Obviously, untreated, these conditions would impact their will.
There is no fraud. There are no conditions placed on consent. There is no error about an essential quality of the person one is about to marry, or obviously an error in regard to the person (marrying the wrong identical twin!).
There is no deceit involved in consent. There is a willingness and capacity to fulfill the obligations of marriage. All of these facets of consent and others are presumed to be healthy and active at the time of consent.
The proceedings surrounding a tribunal investigation circle the consent of both parties and put a question mark on it. The court officials look back in time and raise all kinds of questions about the two ministers. What were their families of origin like? What modeling regarding the role of husband and wife did they receive as children? What was their understanding of marriage at the time of consent? In accord with the mindset of the church, did the ministers intend a lifelong, monogamous union that was open to the gift of children? Were there any conditions placed on their consent? And so forth ... .
What the proceedings attempt to determine is whether any facet of the diamond of consent was defective in one or both of the parties on the wedding day. The presumption is that everything was indeed present. The burden of proof is on the one petitioning for the declaration of nullity.
Any individual has the right to petition the church for a declaration of nullity (cc. 1674 & 1675). That is very different from saying one has a right to a declaration of nullity. One does not have such a right. However, if it can be proven that something essential was lacking in the consent of one or both of the parties, the Church will grant a declaration of nullity. This is the legal theory behind a declaration of nullity.
Next, an example...