We have two issues in play here.
The first is the difference between “grave matter” and “mortal sin.” Or “objective wrong” and “sin.” Every single human act is either objectively wrong or not.
What makes it a sin or not is the combination of our understanding (full knowledge) and our consent. This is the second issue, primacy of conscience.
A person can do an action that is objectively wrong without committing sin if he is unaware that it is wrong or doing it without full consent (being forced, being asleep or partially so, doing it out of fear, etc.).
Conversely, a person can commit a sin by doing an action that is not objectively wrong if he truly believes it to be sinful. (The green bananas thing.)
Here’s an example I have used to explain this. Imagine you wish to hurt someone. You are under the impression that his left shoulder is injured and very tender, so you come up behind him and give him a “friendly” slap on the shoulder, expecting him to recoil in pain. In fact, his right shoulder is the injured one, and you have caused him no pain at all. Have you not by your intention sinned against him?
Singinbeauty thought her father would object to her going through the things in the piano bench, but she did it anyway. The fact that he did not mind does not mitigate the fact that she did what she thought would offend him. Wouldn’t he be hurt if she explained to him that she went through the things thinking he would mind?
Here’s the flip side of the same two examples: Your friend has an injured left shoulder, but you are unaware of it. You give him a friendly slap and he recoils in pain. You are aghast and surprised at having hurt him. Your friend is sore but not offended. This corresponds to doing the objective evil that a person is unaware of.
If Singinbeauty had sincerely believed that her father wanted her to see the things in the piano bench and found out later that he did not, she would not be to blame for doing it. Whether she should have known is not at issue. For the sake of the example, we’ll say she sincerely believed it to be OK.
We often have questions here about confessing things done in the past that a person has only recently discovered are wrong. Without full knowledge, they are not sins. The next time the person does the same thing with full consent, however, it will be sinful because the necessary knowledge is now present.
And the flip side…you do something you sincerely believe to be gravely wrong and later find out that it’s not. You’re not off the hook - you were willing to commit a mortal sin and did the action you believed to be sinful. It needs to be confessed.
Wow - that was a really LONG way of saying I agree with Singinbeauty!
Betsy
P.S. For those who tend toward scrupulosity, forget everything in this post and obey your confessor!!