Theory and practice of the confessional by Caspar Erich Schieler, Richard Frederick Clarke
III. The duty of confessing sins inculpably omitted must be fulfilled either when there is danger of death or at the next confession, whether it be a confession of duty or of choice.
Hence these omitted sins must be confessed, even if no new mortal sin has been incurred, ratione sui when there is grave danger of death and at the time which the Church prescribes for the yearly confession; for the annual confession is prescribed not only in order to obtain sanctifying grace, but also to fulfill the divine law, more clearly defined by the law of the Church. In this case the precept would be binding under grave sin because of the presence of materia necessaria, for a mortal sin omitted even without fault is materia necessaria.
If, however, a confession be made before that time, either of materia necessaria or materia libera, the confession must include the previously omitted sin. This is so evident that no theologian ever dreamt of disputing or doubting it. Every confession must be complete subjectively or formally, and by the declaration of the Council of Trent this confession is not complete unless it includes the sins previously omitted. For this subjective integrity it is required that all mortal sins not yet subjected to the keys which occur to the penitent should be confessed unless some legitimate obstacle stands in the way. If these omitted sins are kept back in the next confession following, that confession is incomplete and sacrilegious. It cannot be argued that these sins had been already indirectly forgiven, for, to speak of no other objection, the same might be urged of sins already condoned by an act of perfect contrition.