Encyclical Letter on the Holy Eucharist Parts 35 & 36

35. The celebration of the Eucharist, however, cannot be the starting-point for communion; it presupposes that communion already exists, a communion which it seeks to consolidate and bring to perfection. The sacrament is an expression of this bond of communion both in its invisible dimension, which, in Christ and through the working of the Holy Spirit, unites us to the Father and among ourselves, and in its visible dimension, which entails communion in the teaching of the Apostles, in the sacraments and in the Church's hierarchical order. The profound relationship between the invisible and the visible elements of ecclesial communion is constitutive of the Church as the sacrament of salvation.⁷¹ Only in this context can there be a legitimate celebration of the Eucharist and true participation in it. Consequently it is an intrinsic requirement of the Eucharist that it should be celebrated in communion, and specifically maintaining the various bonds of that communion intact. 

36. Invisible communion, though by its nature always growing, presupposes the life of grace, by which we become “partakers of the divine nature” (2 Pet 1:4), and the practice of the virtues of faith, hope and love. Only in this way do we have true communion with the Father, the Son and the Holy Spirit. Nor is faith sufficient; we must persevere in sanctifying grace and love, remaining within the Church “bodily” as well as “in our heart”; ⁷² what is required, in the words of Saint Paul, is “faith working through love” (Gal 5:6).

Keeping these invisible bonds intact is a specific moral duty incumbent upon Christians who wish to participate fully in the Eucharist by receiving the body and blood of Christ. The Apostle Paul appeals to this duty when he warns: “Let a man examine himself, and so eat of the bread and drink of the cup” (1 Cor 11:28). Saint John Chrysostom, with his stirring eloquence, exhorted the faithful: “I too raise my voice, I beseech, beg and implore that no one draw near to this sacred table with a sullied and corrupt conscience. Such an act, in fact, can never be called 'communion', not even were we to touch the Lord's body a thousand times over, but 'condemnation', 'torment' and 'increase of punishment'”.⁷³ 

Along these same lines, the Catechism of the Catholic Church rightly stipulates that “anyone conscious of a grave sin must receive the sacrament of Reconciliation before coming to communion”.⁷⁴ I therefore desire to reaffirm that in the Church there remains in force, now and in the future, the rule by which the Council of Trent gave concrete expression to the Apostle Paul's stern warning when it affirmed that, in order to receive the Eucharist in a worthy manner, “one must first confess one's sins, when one is aware of mortal sin”.⁷⁵