In order for a sin to be mortal, there are three requirements:
- Grave matter
- Free consent of the will
- Full knowledge (also called sufficient reflection in the Baltimore Catchism) or knowledge that the action offends God.
It is not necessary to know that the Church labels a particular act or omission “mortal” or “grave.” Some things are discernible through reason alone. For example, anyone may commit mortal sin if they murder someone freely and deliberately, even if they’ve never heard the word “mortal” before.
However, when you say you did not know it was “even a sin” that suggests to me that you did not realize the act offended God. In other words, you did not have sufficient reflection about the sin. Because it was grave matter, it was still sinful, but in your case it would have been venial.
The catechism says:
[quote=]1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law,** or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.**
The guidelines for reception of Holy Communion by Catholics are these:
[quote=]As Catholics, we fully participate in the celebration of the Eucharist when we receive Holy Communion. We are encouraged to receive Communion devoutly and frequently.** In order to be properly disposed to receive Communion, participants should not be conscious of grave sin and normally should have fasted for one hour.** A person who is conscious of grave sin is not to receive the Body and Blood of the Lord without prior sacramental confession except for a grave reason where there is no opportunity for confession. In this case, the person is to be mindful of the obligation to make an act of perfect contrition, including the intention of confessing as soon as possible (canon 916). A frequent reception of the Sacrament of Penance is encouraged for all.
When they say “grave sin” my understanding is that the bishops mean “mortal sin” (not venial sins of a grave matter). This must be the case because communion actually remits venial sin, though I believe you are still obligated to confess all grave matter at your next confession.