No. Generally speaking, the penitent should perform the assigned penance. But neither that particular penance, nor any penance at all, is necessary for the valid absolution of the penitent’s sins. It is a serious doctrinal error to claim that, if the penitent does not accept the assigned penance and resolve to complete it, then this merciful Sacrament of Forgiveness is somehow invalid. “Beware of the leaven of the Pharisees….” (Mt 16:6).
The three parts of the Sacrament of Reconciliation are: 1. contrition, 2. confession (with absolution), 3. satisfaction. These three together are necessary for “the integrity of the sacrament and for the full and perfect remission of sins,” including temporal punishment (Trent, Session 13, chap. 3). However, the principal means of satisfaction for sin in every contrite reception of absolution is the Sacrifice of Christ on the Cross (Trent, Session 6, chap. 7), from which Tree of Life the graces and efficacy of every Sacrament flow. So even if the penitent does not perform the assigned penance, nor any penance at all, he is still forgiven for his sins. Every contrite reception of absolution includes that satisfaction that Christ made for us on the Cross; therefore, every contrite confession includes the third part of the Sacrament, satisfaction.
In addition, the valid absolution of any actual mortal sin always includes satisfaction for the eternal punishment due (Trent, Session 6, chap. 14), apart from the performance of any assigned penance. This satisfaction for eternal punishment is obtained by the penitent’s contrition and confession of his sins, and the absolution of the priest, but its source is the Sacrifice of Christ on the Cross. And this satisfaction for eternal punishment occurs apart from the performance of any assigned penance. Therefore, the part of the Sacrament called satisfaction is not solely or principally found in the acts of assigned penance.
Furthermore, even the temporal punishment that is due for sin is sometimes wholly remitted merely by the devout reception of the Sacrament of Reconciliation, so that any assigned penance is, in some cases, unnecessary even for the remission of the temporal punishment that is always due for sin. For that temporal punishment is sometimes paid in full simply by the devout reception of Confession. The devout reception of any Sacrament always forgives some temporal punishment due for sin. When the Sacrament of Confession is received, if the sins of the penitent are few and venial, it may well be the case that no further temporal punishment remains to be subject to penance. The satisfaction for temporal punishment occurs by the acts of contrition and confession.
In what is unfortunately the more usual case, after a devout reception of Confession, some temporal punishment remains, and therefore the penitent is obliged to perform some penance to make satisfaction for that temporal punishment. If he does not do so, and he dies in a state of grace, then he satisfies that temporal punishment in Purgatory. However, the penitent is free to choose to do either the penance assigned by his confessor, or any penance that he voluntarily chooses as a fitting substitute. Although there is a general obligation to perform the assigned penance, the obligation is not absolute: the penitent can substitute another penance as his own discretion.
But in no case whatsoever does the Sacrament of Confession become invalid merely by the failure, or even the deliberate refusal, of the penitent to perform the particular assigned penance. The sins of the penitent, properly confessed with contrition and absolution, are certainly forgiven, even if much temporal punishment remains without satisfaction of any kind. Such a situation is far from ideal. But the claim that the Sacrament of Reconciliation loses its validity solely by the failure or refusal of the penitent to accept and resolve to perform the assigned penance is a serious doctrinal error, like the errors of the Pharisees. This error is capable of causing grave harm to the spiritual lives of the faithful.
the reproach of Christ 
James, you raise some goood points.
It is contrary to the will of God that all souls be saved for the confessor to withhold absolution, for a penitent who is contrite, even if there is an unresolved issue of justice outside the confessional. Also, submitting to civil authorities (when one is guilty of a serious crime from which one has repented) might not be moral. Under the eternal moral law all three fonts of morality must be good for any act to be moral. If submitting will have grave bad consequences that are not outweighed by good consequences, such as harm to a man’s wife and children, and harm to him if civil authorities in the nation are known to give excessive penalities for whatever the crime was, then submitting would not be moral.
If a penitent refuses to do any penance, for a grave sin this refusal is an objective mortal sin (but it might not be an actual mortal sin), and for a venial sin, this refusal is only venial. So the penitent does sin anew if he refuses to do any penance.
However, the penitent can substitute his own voluntarily chosen penance without sinning anew. The eternal moral law requires penance, but it does not require the exact penance specified by the confessor, or by Canon Law. To say otherwise is to adhere to the type of Pharisaism condemned by Scripture:
“All is lawful to me, but not all is expedient.” (1 Cor 6:12)
“For you are not under the law, but under grace.” (Rom 6:14)
” ‘Have you not read what David did, when he was hungry, and those who were with him: how he entered the house of God and ate the bread of the Presence, which was not lawful for him to eat, nor for those who were with him, but only for the priests? Or have you not read in the law, that on the Sabbaths the priests in the temple violate the Sabbath, and they are without guilt?’ ” (Mt 12:3-5)
Claims about what Council Fathers said or debated outside of Conciliar decisions is irrelevant. This claim is often used to undermine the teachings of a Council (such as Vatican II) or of a Pope (e.g. Humanae Vitae). The teachings of each Council are acts of the Magisterium. Private discussions prior to such acts are not of the Magisterium.
See my next post on this subject later today.
What about a case when a mortal sin also transgresses a just civil law for which the penitent also owes reparation in justice and charity? It would seem that moral theology allows that a priest may withhold absolution if the penitent does not agree to submit to the authorities and penalties of civil law; being that the seal of confession is absolute, yet the priest having an obligation to impose a penance that is serious and salutary to both the penitent and in some cases the victim. I am thinking of murder when the murderer has escaped detection.
In reading Moral and Pastoral Theology by Henry Davis, S.J. I read that according to the old Canon Law 887: The penitent must willingly accept and personally fulfill the penance imposed. The double obligation is of its nature is grave in a grave matter and light in a matter that is not grave. Deliberately to neglect to fulfill a grave penance, or a notable part of it is a serious sin. Let me know what you think.
I am certainly not contradicting Trent or Aquinas on the grace of the sacrament ex opere operato. My distinction is not that the sacrament is invalid. Rather, the penitent sins anew upon refusing to accept the penance.
I also read that there was an apparent debate between the Council of Florence and Trent as to whether the acts of the penitent should be considered quasi materia necessary for the integrity of the sacrament.
James