Hi Thomas,

Sorry for not getting back to you sooner.

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No, you may not. If one is aware of grave, meaning mortal sin one has committed, they have to first go to Confession before they can receive Holy Communion at Mass. Confessions are scheduled at most, if not all, Catholic parishes every Saturday afternoon, before the Saturday Vigil Mass.

I hope this answers your question.

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Mike

Thomas,

Let me just clarify what my colleague Mike has said a bit.

First, let's define mortal sin completely. For a sin to be a mortal sin, it must meet (3) three conditions.

1. Grave matter. It must be objectively grave matter.  That essentially means it somehow violated one of the 10 Commandments, and that includes sins related to them.
   
   For example, the 6th Commandment prohibits adultery. Strictly speaking, adultery is illicit sexual conduct where one or both of the parties involved are married to someone else. 
   
   But it also includes many other sexual sins.  Sex outside of (or before) marriage, the use of artificial birth control, any lustful behavior, in thought or action, alone or with someone else, homosexual acts, pornography, etc.
   

All of those violate the 6th Commandment and such sin is considered grave matter.

The other 2 conditions for a sin to be a mortal sin are more subjective.

2. Full knowledge: Someone has to reasonably understand that an action is sinful, or the Church Teaches it's a sin.   So, for example, if someone was never taught that deliberately missing Mass, without a legitimate reason is a grave sin, then their culpability is mitigated.
   
   
3. Full Consent of the Will: . . . with sufficient time to reflect upon your action before you act.
   
   This is perhaps the most subjective.    If you willfully rob a bank, or commit fraud knowing it's a grave sin, you have time to think about it, and you still do it, then it's more than likely mortal sin.
   

However, if you steal food because your family is hungry, you're desperate, and don't have a clear head, then again, your culpability is less. 

If someone is holding your child hostage until you rob a bank, that's not full consent of the will. 

I went through this explanation so that it’s clear.  All issues of grave matter are not necessarily mortal sins. But mortal sins are always grave matter. however, if we are conscious of grave matter, we must abstain from receiving the Eucharist, because it's not up to us to judge whether the circumstances of full knowledge and full consent are met.  That's something God knows for sure. 

We can have a pretty good idea, but we can't know for sure so we are required to confess all sins that are of grave matter, regardless of whether we think we have (or have not) given our full consent of the will.