In Canon Law (# 916) we are reminded that a person “conscious of grave sin” should not receive the Eucharist without previous sacramental confession.” We should examine our conscience before coming to Mass to discern whether we are aware of having committed a mortal (serious) sin. If so, we should have recourse to the Sacrament of Penance—the normal means through which sins are forgiven and we are reconciled with God—so that we may worthily share in the Eucharist.
This Canon is rooted in the Church’s tradition and in Sacred Scripture. We read in St. Paul’s First Letter to the Corinthians: “Whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord. A person should examine himself, and so eat the bread and drink the cup. For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself. That is why many among you are ill and infirm, and a considerable number are dying.” (11:27-30)
Receiving Communion knowingly in a state of mortal sin is a grave sin in and of itself. Specifically, it is a sin of disrespect (sacrilege) for the Real Presence of Christ in the Eucharist which is a wound to the soul. Of course, the Eucharist is such an awesome gift that no one could ever be considered truly “worthy” in the strictest sense of the word. But at the same time, it’s important to keep in mind that only those who are in mortal sin should refrain from receiving Communion.
The Church encourages those who are able to receive Holy Communion to do so as often as possible, even daily (CCC 1389). For those who are properly disposed, Holy Communion can “wipe away venial sins” (CCC 1394), and provide the strength to resist the temptation to mortal sin (CCC 1395). This is what is meant when we say that the Eucharist “is not a prize for the perfect, but a powerful medicine and nourishment for the weak.” (cf. Amoris Laetitia footnote 351)