Encyclical on the Holy Eucharist - Parts 37 & 38

37. The two sacraments of the Eucharist and Penance are very closely connected. Because the Eucharist makes present the redeeming sacrifice of the Cross, perpetuating it sacramentally, it naturally gives rise to a continuous need for conversion, for a personal response to the appeal made by Saint Paul to the Christians of Corinth: “We beseech you on behalf of Christ, be reconciled to God” (2 Cor 5:20). If a Christian's conscience is burdened by serious sin, then the path of penance through the sacrament of Reconciliation becomes necessary for full participation in the Eucharistic Sacrifice. 

The judgment of one's state of grace obviously belongs only to the person involved, since it is a question of examining one's conscience. However, in cases of outward conduct which is seriously, clearly and steadfastly contrary to the moral norm, the Church, in her pastoral concern for the good order of the community and out of respect for the sacrament, cannot fail to feel directly involved. The Code of Canon Law refers to this situation of a manifest lack of proper moral disposition when it states that those who “obstinately persist in manifest grave sin” are not to be admitted to Eucharistic communion.⁷⁶

38. Ecclesial communion, as I have said, is likewise visible, and finds expression in the series of “bonds” listed by the Council when it teaches: “They are fully incorporated into the society of the Church who, possessing the Spirit of Christ, accept her whole structure and all the means of salvation established within her, and within her visible framework are united to Christ, who governs her through the Supreme Pontiff and the Bishops, by the bonds of profession of faith, the sacraments, ecclesiastical government and communion”.⁷⁷ 

The Eucharist, as the supreme sacramental manifestation of communion in the Church, demands to be celebrated in a context where the outward bonds of communion are also intact. In a special way, since the Eucharist is “as it were the summit of the spiritual life and the goal of all the sacraments”,⁷⁸ it requires that the bonds of communion in the sacraments, particularly in Baptism and in priestly Orders, be real. It is not possible to give communion to a person who is not baptized or to one who rejects the full truth of the faith regarding the Eucharistic mystery. Christ is the truth and he bears witness to the truth (cf. Jn 14:6; 18:37); the sacrament of his body and blood does not permit duplicity.