Catechism of the Catholic Church
c, d, and e. Para. 1415: Anyone who desires to receive Christ in Eucharistic communion must be in the state of grace. Anyone aware of having sinned mortally must not receive communion without having received absolution in the sacrament of penance.
Para. 1394: As bodily nourishment restores lost strength, so the Eucharist strengthens our charity, which tends to be weakened in daily life; and this living charity wipes away venial sins (Cf. Council of Trent [1551]: DS 1638). By giving himself to us Christ revivies our love and enables us to break our disordered attachments to creatures and root ourselves in him . . .
Para. 1395: By the same charity that it enkindles in us, the Eucharist preserves us from future mortal sins. The more we share the life of Christ and progress in his friendship, the more difficult it is to break away from him by mortal sin. The Eucharist is not ordered to the forgiveness of mortal sinsthat is proper to the sacrament of Reconciliation. The Eucharist is properly the sacrament of those who are in full communion with the Church.
Para. 1417: The Church warmly recommends that the faithful receive Holy Communion when they participate in the celebration of the Eucharist; she obliges them to do so at least once a year.