Once a priest always a priest (see Ps. 110:4); however, there is a delicate distinction that must be made between the metaphysical fact that a man is always a priest once he has been ordained, and the canonical status of a laicized priest. Canon law is in complete accord with theology on this subject. The Catechism states that the sacrament of Holy Orders confers an āindelible spiritual characterā on the man who receives it (CCC 1582). Like the sacrament of Baptism, it can never be erased. A baptized Christian can cease to practice his faith, and even publicly deny Christ, but he can never undo his baptism. Priestly ordination works in exactly the same way.
Similarly, canon 290 of the Code of Canon Law states once a man validly receives sacred ordination, the sacrament never becomes invalid. At the same time, however, it is possible for a priest to be released from the duties and responsibilities that are connected to the clerical state (CCC 1583). Practically speaking, this would mean that a priest no longer functioned outwardly as a priest. He would no longer engage in ministry within his diocese or religious institute; no longer celebrate Mass or confer the sacraments; no longer be called āFatherā or wear clerical clothing; and no longer be supported financially by the Church. To the world he would appear to be a layman, working at an ordinary job and living the normal life of the laity. Canon law refers to this change as the āloss of the clerical stateā (cf. cc. 290-293). Common parlance calls it laicization. When this occurs, and a priest is released from the clerical state, he is still technically a priest, but as canon 292 notes, he may no longer exercise the power of orders. But in theory, if a laicized priest were to say Mass, it would be a valid Mass, since he never loses the ability to celebrate the Eucharist. It would, however, be illicit. It is possible for a priest to receive permission to be once more āre-instatedā directly from Rome (c. 293) and would not be ordained again, as he would still be an ordained priest.