To further affirm Shelb's point that to receive the Eucharist we are to believe in the teachings of the Church (in their entirety), let me post this snippet from something that is called an Encyclical.
From Ecclesia de Eucharistia
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
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.
39. Furthermore, given the very nature of ecclesial communion and its relation to the sacrament of the Eucharist, it must be recalled that the Eucharistic Sacrifice, while always offered in a particular community, is never a celebration of that community alone. In fact, the community, in receiving the Eucharistic presence of the Lord, receives the entire gift of salvation and shows, even in its lasting visible particular form, that it is the image and true presence of the one, holy, catholic and apostolic Church.
79 From this it follows that a truly Eucharistic community cannot be closed in upon itself, as though it were somehow self-sufficient; rather it must persevere in harmony with every other Catholic community.
The ecclesial communion of the Eucharistic assembly is a communion with its own
Bishop and with the
Roman Pontiff. The Bishop, in effect, is the
visible principle and the foundation of unity within his particular Church.
80 It would therefore be a great contradiction if the sacrament
par excellence of the Church's unity were celebrated without true communion with the Bishop. As Saint Ignatius of Antioch wrote: That Eucharist which is celebrated under the Bishop, or under one to whom the Bishop has given this charge, may be considered certain.
81Likewise, since the Roman Pontiff, as the successor of Peter, is the perpetual and visible source and foundation of the unity of the Bishops and of the multitude of the faithful,82 communion with him is intrinsically required for the celebration of the Eucharistic Sacrifice. Hence the great truth expressed which the Liturgy expresses in a variety of ways: Every celebration of the Eucharist is performed in union not only with the proper Bishop, but also with the Pope, with the episcopal order, with all the clergy, and with the entire people. Every valid celebration of the Eucharist expresses this universal communion with Peter and with the whole Church, or objectively calls for it, as in the case of the Christian Churches separated from Rome.83