Receiving Holy Communion, while in a state of mortal sin, is a sin against the Lord

In the Holy Eucharist, Jesus is giving Himself to us in the form of bread and wine. The Lord is truly present in that tiny white host - Body, Blood and Divinity. Hence, it is important that when we receive Him in Holy Communion, we are under the state of grace. In other words, we are not in the state of mortal sin, or, if we currently are, we must confess our sins first and receive absolution in the Sacrament of Reconciliation. 

The Holy Scriptures and the Catechism of the Catholic Church preaches that whoever receives the Lord unworthily is guilty of profaning the Body and Blood of the Lord.

1. The Apostle Paul warns us “Therefore whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord.” (cf. 1 Corinthians 11:27) 
2. The Catechism also warns us “Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession. Children must go to the sacrament of Penance before receiving Holy Communion for the first time.”  -Catechism of the Catholic Church 1457