That said, "Extra ecclesiam nulla salus" asked the following question in the Q&A board at Phatmass:
If one does something that one thinks is a mortal sin and it is not even a sin does the person commit a mortal sin?Short answer: No.
We have to keep in mind that actions have an objective quality, irrespective of how you may subjectively feel about them. So, some sins are grave even though you may feel they are not, and other sins are venial even tho you may feel they are more serious. Likewise, some actions are not sinful at all even though you may feel that they are.