I have always been told to confess the mortal sins forgotten (and later remembered): this is the reason I have said you’re supposed to do it. Ask an Apologist on this site also routinely mentions that we should confess the forgotten mortal sins when we remember them, but I don’t recall reading the justification. I have been asuuming the reason is canon law since the Fundamentals of Catholic Dogma by Ludwig Ott cites canon law when it says we have the obligation (pg 433):
The citation Ott gives after what I have typed in is the CIC, but it is the older one, not the latest one, so I can’t really look it up. The canon I did find and mention from the newer CIC uses the words
directly remitted, so I assumed it was relevant that a forgotten sin is indirectly remitted (what I put in red above). I guess I’m saying that these forgotten sins seem to be all of these four simultaneously A) grevious and after baptism B) only indirectly remitted C) not mentioned in individual confession D) they have been remembered this time when doing the examination. But I only thought the middle two relevant to highlight.
I am not a canon lawyer, however. It is true I may be misunderstanding it, but it is also true that I have been repeatedly told one is supposed to confess those sins.
See the Ask an Apologist:
Fr Vincent Serpa