The wonderful effects of Holy Communion are not automatic. They demand from the communicant adequate preparation. The most important form of preparation is that the person be in the state of grace. It would be a sacrilege to receive Jesus while one is in the state of mortal sin. A person in mortal sin should first repent, go to confess to a priest who will give absolution in the name of Christ and the Church, and only then come to the Eucharistic table…Pope Benedict XVI…also underlined the role of the sacrament of Penance even for people who have no grave sins because this sacrament strengthens our continued effort to follow Christ with growing fidelity. Saint Paul urges us to examine our conscience before we receive Holy Communion: “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 anyone who eats and drinks without discerning the Body eats and drinks judgment upon himself.” (1 Cor 11: 27-29)
Francis Cardinal Arinze: Celebrating the Holy Eucharist, pp. 21-22