Sure
sins can be “absolved” …when forgotten…indirectly as it were…but then one still needs to “submit them to the keys” in the next confession (unless you forget hehe)
Canon law…
Can. 988 §1. A member of the Christian faithful is obliged to confess in kind and number all grave sins committed after baptism and not yet remitted directly through the keys of the Church nor acknowledged in individual confession, of which the person has knowledge after diligent examination of conscience.
Every bit of the this canon is important…
here are a few notes:
- We are Obliged to confess in number and kind all grave sins (meaning mortal sin…these terms are used interchangably by the Church)
- It is both number and kind (murder 3 times)
- That were not directly remitted…that means directly confessed and absolved “I murdered 3 persons”)…when a mortal sin is forgotten (not hidden) it is “indirectly absolved” thus it still needs to be confessed “directly” in number and kind in the next confession (even though it was indirectly absolved)
- and again there is further emphasis that they need to be confessed in an individual confession…not only absolved in an emergency without confession (like they are dying and have no time…but then recover…or the plane is going down and the priest gives general absolution (legitimately in that case) and then they survive…they still need to confess any mortal sins)…or indirectly…as in forgotten mortal sins…
So if someone unknowingly omits a mortal sin…no problem it was indirectly absolved…but they still have the obligation (within reason…our memories can not serve us…etc) to confess them…(again those with scruples talk with your confessors…for this can be a source of scruples…historically in pastoral practice they can even be an exception to this…due to their scruples…and they often start scrupling about it…so I encourage them not to and to submit to their confessor etc)
Also if something “changes” the nature of the sin…like it was your sister that you killed …then that too would be part of the confession…but not scrupulous details…
(by the way I do have a degree in Theology from Steubenville…and did very well in Sacraments…the Dominican even stopped me in the hall “you did not get even one thing wrong!”…after the exam…I mention this …so you can rest assured

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