In Catholic theology, only one bishop is necessary for a valid consecration of another bishop. However, the Church, since ancient times, has always required the active participation of two other bishops (called Co-consecrators) in the ordination ceremony of a new bishop. There are many reasons that account for this lawful requirement. It suffices to give here two of them: theological and historical.
The Theology
The theological reason has to do with what we call collegiality, which is to say, no bishop is an island in and of himself. It is true that a bishop has jurisdiction over a particular place (diocese), but also, by his ordination, he becomes part of the college of bishops, the successors of the apostles.
Thus, he acts in communion with other bishops, under the authority of the pope. It is precisely due to this collegiality that he is mentioned in the Eucharistic prayers at Mass immediately after the pope, expressing this union with other bishops and the pope, who is the head of the college of bishops.
Co-consecration (consecration of a new bishop by three bishops), therefore, is an outward sign of the new bishop’s connection with his fellow bishops in the episcopate. He is dependent upon them just as the Apostolic Band (of the Twelve) depended on each other as close collaborators of the Lord.
Co-consecration is a reminder to the new bishop that his ministry and his diocese are linked with the ministry of other bishops and other dioceses.
More recently, the existence of national and regional Bishops’ Conferences is a visible expression of this collegiality among bishops—that no bishop is an island; he is a bishop in the one, holy, catholic, and apostolic Church.
This unity and catholicity of the churches (dioceses) is also a recurring theme in the rite of consecration of a bishop. St. Paul also expresses this in his teachings on the unity of the body of Christ (cf. Ephesians 1:4-6).
The History
The history comes from the frequent periods in the early Church when heresies were common, especially in the fourth century, a time of intense division and struggle for control in the Church.
During such times, some heretical bishops made it their business to multiply episcopal ordinations in order to have more bishops as political allies.
Thus the requirement of three bishops at the consecration of a new bishop was a way of ensuring the validity of the consecration action. With three bishops, the intention of the consecrator (for the good of Mother Church) was secured.
Co-consecration was formally prescribed in the year 325 at the First Ecumenical Council of Nicaea, which decreed: “a bishop ought to be chosen by all the bishops of his province, but if that is impossible because of some urgent necessity, or because of the length of the journey, let three bishops at least assemble and proceed to the consecration, having the written permission of the absent.”
The compulsory co-consecration thus forced neighbouring bishops, even if they were not allies in the controversies of their time, to come together for the consecration of a new bishop, thus eliminating any hidden agendas.
For these and other reasons, when we attend the consecration of a new bishop today, we see three bishop-consecrators (a principal consecrator and two co-consecrators), reminding us of the universality of the episcopal office and also of the long history behind the development of the office of apostolic succession from its earliest times.
