[quote="Zenkai, post:1, topic:305263"]
Scenario: Someone did something and didn't realize it was a sin, therefore they didn't commit a sin at all because they didn't have full knowledge, etc. Then one day they found out that it was indeed a sin. So, would this person have to confess this even if they didn't know it was a sin at the time it was committed?
I don't think they would have to confess it because they didn't have full knowledge and therefore didn't commit a sin. So, there is no sin to confess.
Am I correct?
You are correct. For something to be a mortal sin (which must be confessed), it has to involve grave matter, you have to have full knowledge of its sinfulness, and you have to give full consent of the will to the sin. If any one of these was missing, the sin was not mortal and need not be confessed. However, it is still objectively sinful (albeit a lesser, venial sin due to the circumstances). So it's not that there is no sin to confess, it's that there's no MORTAL sin to confess.
You still CAN confess venial sins, even though you are not obligated to do so. In my own life, I did many sinful things before I was a Catholic. Some of which I knew were sinful, but many of them I had no idea. I still brought them all (as best as I could remember them) to my first confession. Not because I had to, but because I still felt guilt for them and wanted to 'come clean.' I felt like saying 'I didn't know better,' although it was true, would be letting myself off too easy ;-). But that's just a personal decision on my part.