Can I receive communion if I didn't know I did a mortal sin?

A few months ago I confessed a mortal sin. At the time I didn’t know it was mortal so I assumed it was venial because of my ignorance and received Communion until I was able to go to confession(which was months after I committed the sin due to the fact I’m still in school and had to wait until the a priest came to my school for confessions). During confession I said what I did but didn’t know it was mortal at the time I committed it. I forgot to say that I received Communion. I thought since I didn’t know how serious my sin was that it knocked it down to a venial sin. Am I right?

To be guilty of a mortal sin 3 conditions must be met (CCC 1857) :

  1. grave matter
  2. know its grave matter
  3. freely choose it

In the situation you describe conditions 1 & 3 are present but not 2. Therefore you were not guilty of a mortal sin. Once you realized it was grave matter you confessed it. Under this scenario you didn’t do anything wrong by receiving communion.

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