Canonization, the ceremony by which a deceased person, who has previously been beatified (see Beatification), is proclaimed a saint in the Roman and Greek churches. In the Roman church this is done by the pope, who, after investigation, declares the person in question to have led a perfect life, that God has worked miracles at his intercession, either during his life or after his death, and that consequently he is worthy to be honored as a saint. In the Greek church canonization is performed by the patriarch and the bishops assembled in synod, and requires the testimony of a thousand witnesses to the virtues of the deceased. The trouble and expense of this process are so great that few saints are canonized in the Greek church. Before a beatified person can be canonized in the Roman church four consistories must be held. In the first the pope causes the petition of the parties requesting the canonization to be examined by cardinals appointed for the purpose; in the second the cardinals report the result of their investigation; in the third, which is public, a person called the devil's advocate (advocatics diaboli) says all he can against the proposed saints, to which another advocate responds by praising him, and reciting the miracles he has performed; in the fourth and last consistory, at which all the cardinals are convened, the canonization is put to the vote, and if the verdict is favorable the person is pronounced a saint.

The first canonization is said to have been performed by Leo III. in 804. No person can be canonized until 50 years after death, except in cases of martyrdom.