Question: What exactly does a priest do when he “offers Mass for someone’s intentions.”
Answer: There are a number of benefits (usually called “fruits”) gained whenever Mass is offered. The “general fruits” of the Mass are applied by the Church to all of Her members, living and dead, unless they place some obstacle to receiving them. Thus, each Mass petitions God for the needs of the faithful and the forgiveness of their sins. This offering is independent of the will of the priest.
The “special fruits” of the Mass are applied to all who, in some manner, participate in the offering of Mass. Participation ranges from mere bodily presence, through making the responses, up to those who actually minister at the altar. The fruits received are proportional to the degree of participation, adding satisfaction for sin to the graces of the general fruits.
The “personal fruits” of the Mass are the “special fruits” garnered by the celebrating priest himself, who, of course. participates in the highest degree possible for a mere human being.
The “ministerial fruits” of the Mass are those which apply to the particular person(s) or object for whom the priest offers Mass. They include petition for his (their) needs, a request for the forgiveness of sins, and satisfaction of temporal punishment, much like the “special” or “personal” fruits.
The priest is expected to formulate an intention at each Mass as to the distribution of the ministerial fruits. This intention may be quite specific: e.g. for the repose of the soul of Mary Smith; for an end to legal abortion; or for the good health of Mr. and Mrs. Jones. On the other hand, the priest may simply offer Mass in accord with the (unknown to the priest) intentions of a person or group, the intention recorded in his schedule book, the intention of his superior or a stipend donor, the person most tempted to sin, or the most abandoned soul in Purgatory—for even if the exact intention is not known to the priest, the appropriate recipients of the ministerial fruits are known in the mind of God. Most priests formulate a conditional intention, so that if for some reason the primary intention is not valid, a second intention is applied—often this is to leave the ministerial fruits to the disposition of the Blessed Virgin Mary.
While it might be adequate to have the intention written in his appointment book, the conscientious priest will always make the attempt to formulate and express the intention as he prepares for Mass and in Its celebration. The Roman Missal has a written “Declaration of Intention,” which the priest may make his own:
This writer adds a few more specific things:
The priest may, on some days, be able to include a special Collect, Secret, and Postcommunion prayer appropriate to his ministerial intention. These prayers, collectively called “orations” are found toward the back of the Roman Missal. Many of the orations for the dead allow the priest to insert the deceased persons name(s). There are also orations which may be offered for a number of other intentions—the health of the sick, for peace, for good weather, for friends, for family, etc. Such orations may be added even if they are not in accord with the intention for the ministerial fruits. That is, the priest may add orations for (say) peace, or for rain, even though the ministerial fruits are assigned to some private intention.
Finally, in the Canon of the Mass there are places where it is appropriate to pause and silently remember the names of the living and the dead for whom the priest wishes to pray—for the living, before the consecration; for the dead, following it. The priest is free to remember whom he will, even those who are not the object of his ministerial intention.
It is a holy and a wholesome though to pray for the dead, that them might be loosed from their sins. It is praiseworthy to have Mass offered for those in need of Its great benefits—both the living and the dead. If at all possible one ought to assist at the Masses one has offered.