The Catechism of the Summa - Tertia Pars - XXXIX. OF THE PART OF THE PENITENT IN THE SACRAMENT OF PENANCE; OF CONTRITION, CONFESSION, AND SATISFACTION (D)

(D)
What must the sinner do after having conceived sorrow for his sins, whether this sorrow be attrition or contrition?

He must go to the priest and confess his sins (VI. 1-5).

When does the Church oblige one to confess?
For all the faithful once a year, at Easter time by preference, by reason of the Easter communion which no one may receive if he have mortal sin on his soul (VI. 5; Code, Canon 906).

Why is confession necessary in order to receive the sacrament of Penance?

Because it is only by confession that the penitent can make known his sins to the priest, whose duty it is to judge whether the penitent is worthy to receive absolution, and to impose some Penance as satisfaction for the sins committed, in order that a just compensation might be offered to God for the renewal of His grace (VI. 1).

What does confession entail for the sacrament to be valid?

As far as possible the sinner must make known to the priest in detail the number and the species of mortal sins committed; and he must confess them with a view to obtaining the sacramental absolution from the priest (IX. 2).

If at the moment of self-accusation the sinner lack both contrition and attrition for his sins, does the absolution given by the priest remit the sins?

No, they are not remitted; but they would be confessed, and there would be no need of repeating them to the priest again in order for them to be remitted by the virtue of the sacrament; it is sufficient for the sinner to conceive contrition for them and to accuse himself in his next confession of the want of contrition in his preceding confession (IX. 1).

If one has forgotten to confess some mortal sin in confession, and afterwards one remembers, is one bound to confess this sin in the next confession?

Yes, because every mortal sin must be submitted directly to the power of the keys (IX. 2).

In what capacity does the priest receive the confession of the sinner?

He receives it in the name and in the place of God Himself; and this in such wise that in his life as man, and outside his ministry as confessor, he knows nothing whatever of the sins confessed to him (XI. 1).

Next - The Catechism of the Summa - Tertia Pars - XXXIX. OF THE PART OF THE PENITENT IN THE SACRAMENT OF PENANCE; OF CONTRITION, CONFESSION, AND SATISFACTION (E)