**Before delving into this matter more deeply, it’s important to note that the Code of Canon Law says absolutely nothing about this phenomenon. That, in itself, is not surprising—because by its very nature, canon law presumes good faith on the part of those whom it binds. Put differently, the Church assumes that its members want to be good Catholics, and to do what the Church teaches is correct and just. The whole purpose of canon law is to clarify matters, so as to enable us Catholics to make right choices and to act correctly. Consequently it does not directly address the legal implications of the actions of renegade Catholics who freely and knowingly walk away from the Church, other than to make it clear that they shouldn’t do this!**
Very telling.
First of all, as was already discussed in greater detail in “Can a Priest Ever Return to the Lay State?” once a man is ordained a priest, his priesthood never becomes invalid (c. 290). This is a legal concept firmly rooted in Catholic theology: the Church teaches that ordination—like the sacraments of baptism and confirmation—confers a permanent ontological change on the person receiving it. That’s why these sacraments cannot be received more than once (c. 845.1; cf. also Catechism of the Catholic Church 1582).
It logically follows that once a man is validly ordained a priest, he will always be a priest, and (as per canon 1338.2) no one on earth can take that away from him! Nevertheless, it is canonically possible for an ordained Catholic priest to return to the lay state: Rome can, if it so decides, release a priest from the duties and obligations which are connected with being a cleric (see “Can a Priest Ever Return to the Lay State?” for more on this). Sometimes this is done because the priest requests it; other times, as discussed in “What Does it Mean to Defrock a Priest?” it is imposed as a penalty—but regardless of who initiates it, the end-results are canonically the same.
One result, which is relevant to Patrick’s question, can be found in canon 292, which notes that a priest who has lost the clerical state is prohibited from exercising holy orders. This means, of course, that he must not administer the sacraments, is not permitted to preach, and may not bless anyone/anything. The canon observes that there is only one exception to this rule: in accord with canon 976, a laicized priest is able—and in fact is obliged—to hear the confession of a person in danger of death who requests it. This is because the spiritual well-being of a dying person “trumps” the laicized priest’s obligation to refrain from priestly ministry. (See “Can All Priests Always Hear Confessions?” for more on this scenario.) But apart from this uncommon situation, a priest who has returned to the lay state is not permitted to celebrate the sacraments. Once he has been laicized he is, in short, supposed to be living his life as any other member of the laity.