Question: Is it really a sin to receive the Eucharist in mortal sin?
No, it isn’t simply a mortal sin but a form of sacrilege; it is worse than a mortal sin to receive Jesus in mortal sin. The once life-giving body and blood of Christ becomes judgement to the soul. It is important to realize that we mustn’t rise with everyone every time they’re going to receive the Eucharist. We have to carefully examine ourselves. Thankfully, a sound conscience doesn’t need much examination to realize whether or not they’re in the state of grace. Here’s what the Church says about receiving without preparation:
To respond to this invitation we must prepare ourselves for so great and so holy a moment. St. Paul urges us to examine our conscience: “Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For any one who eats and drinks without discerning the body eats and drinks judgment upon himself.” Anyone conscious of a grave sin must receive the sacrament of reconciliation before coming to Communion
– Catechism of the Catholic Church 1385.