Objection 1. It would seem that it is not necessary for confession to be entire, namely, for a man to confess all his sins to one priest. For shame conduces to the diminution of punishment. Now the greater the number of priests to whom a man confesses, the greater his shame. Therefore confession is more fruitful if it be divided among several priests.
Objection 2. Further, confession is necessary in Penance in order that punishment may be enjoined for sin according to the judgment of the priest. Now a sufficient punishment for different sins can be imposed by different priests. Therefore it is not necessary to confess all one's sins to one priest.
Objection 3. Further, it may happen that a man after going to confession and performing his penance, remembers a mortal sin, which escaped his memory while confessing, and that his own priest to whom he confessed first is no longer available, so that he can only confess that sin to another priest, and thus he will confess different sins to different priests.
Objection 4. Further, the sole reason for confessing one's sins to a priest is in order to receive absolution. Now sometimes, the priest who hears a confession can absolve from some of the sins, but not from all. Therefore in such a case at all events the confession need not be entire.
On the contrary, Hypocrisy is an obstacle to Penance. But it savors of hypocrisy to divide one's confession, as Augustine says [De vera et falsa Poenitentia, work of an unknown author]. Therefore confession should be entire. Further, confession is a part of Penance. But Penance should be entire. Therefore confession also should be entire.
I answer that, In prescribing medicine for the body, the physician should know not only the disease for which he is prescribing, but also the general constitution of the sick person, since one disease is aggravated by the addition of another, and a medicine which would be adapted to one disease, would be harmful to another. The same is to be said in regard to sins, for one is aggravated when another is added to it; and a remedy which would be suitable for one sin, might prove an incentive to another, since sometimes a man is guilty of contrary sins, as Gregory says (Pastoral. iii, 3). Hence it is necessary for confession that man confess all the sins that he calls to mind, and if he fails to do this, it is not a confession, but a pretense of confession.
Reply to Objection 1. Although a man's shame is multiplied when he makes a divided confession to different confessors, yet all his different shames together are not so great as that with which he confesses all his sins together: because one sin considered by itself does not prove the evil disposition of the sinner, as when it is considered in conjunction with several others, for a man may fall into one sin through ignorance or weakness, but a number of sins proves the malice of the sinner, or his great corruption.
Reply to Objection 2. The punishment imposed by different priests would not be sufficient, because each would only consider one sin by itself, and not the gravity which it derives from being in conjunction with another. Moreover sometimes the punishment which would be given for one sin would foster another. Again the priest in hearing a confession takes the place of God, so that confession should be made to him just as contrition is made to God: wherefore as there would be no contrition unless one were contrite for all the sins which one calls to mind, so is there no confession unless one confess all the sins that one remembers committing.
Reply to Objection 3. Some say that when a man remembers a sin which he had previously forgotten, he ought to confess again the sins which he had confessed before, especially if he cannot go to the same priest to whom his previous confession was made, in order that the total quantity of his sins may be made known to one priest. But this does not seem necessary, because sin takes its quantity both from itself and from the conjunction of another; and as to the sins which he confessed he had already manifested their quantity which they have of themselves, while as to the sin which he had forgotten, in order that the priest may know the quantity which it has under both the above heads, it is enough that the penitent declare it explicitly, and confess the others in general, saying that he had confessed many sins in his previous confession, but had forgotten this particular one.
Reply to Objection 4. Although the priest may be unable to absolve the penitent from all his sins, yet the latter is bound to confess all to him, that he may know the total quantity of his guilt, and refer him to the superior with regard to the sins from which he cannot absolve him.
Source: Summa Theologica (New Advent)