From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
The definition which St. Augustine gives of the eucharist as “sacramentum pietatis, signum unitatis, vinculum caritatis“155 clearly illustrates the effects of personal sanctification (pietas) and community reconciliation (unitas and caritas) which derive from the very essence of the eucharistic mystery as an unbloody renewal of the sacrifice of the cross, the source of salvation and of reconciliation for all people.
However, it must be remembered that the church, guided by faith in this great sacrament, teaches that no Christian who is conscious of grave sin can receive the eucharist before having obtained God’s forgiveness. This we read in the instruction Eucharisticum Mysterium which, duly approved by Paul VI, fully confirms the teaching of the Council of Trent: “The eucharist is to be offered to the faithful also ‘as a remedy, which frees us from daily faults and preserves us from mortal sin’ and they are to be shown the fitting way of using the penitential parts of the liturgy of the Mass. The person who wishes to receive holy communion is to be reminded of the precept: Let a man examine himself” (1 Cor 11:28). And the church’s custom shows that such an examination is necessary, because no one who is conscious of being in mortal sin, however contrite he may believe himself to be, is to approach the holy eucharist without having first made a sacramental confession. If this person finds himself in need and has no means of going to confession, he should first make an act of perfect contrition.156
155 St. Augustine, In Ioannis Evangelium Tractatus, 26, 13: Corpus Christianorum, Latin series 36, 266.
156 Sacred Congregation of Rites, Instruction on the Worship of the Eucharistic Mystery Eucharisticum Mysterium (May 25, 1967), 35: Acta Apostolicæ Sedis 59 (1967), 560f.
— Reconciliatio et Pænitentia 27