The Eucharist is the communion of the body and blood of the Lord. As St. Paul explains, it ought to be taken in order that we might abide and live more fully in Christ and he might live and abide more fully in us. For this reason St. Paul stresses that in the celebration of the Holy Supper we ought increasingly to desire to live and abide in Christ (i.e., to eat the flesh and drink the blood of the Lord) and to receive this meat and drink with greater fruitfulness and religious fervor. Hence, it is necessary to institute and to control this sacrament in order that the people might be duly instructed and admonished as to the necessity of their frequent participation in the flesh and blood of the Lord as well as to its great benefits, which are received from this participation and mastication.
From this it follows, first, that the Supper ought only to be offered to those who are willing and able to live in the Lord, who have him living in them, and who desire that his life be increased and made greater in them. For the reason why the communion of the body and blood of Christ is given in the Holy Supper is to the end that we might live entirely in him and he in us. Consequently, it is necessary for God's faithful ministerial dispensers to know that those to whom they wish to give the Lord's Supper are already incorporated by baptism into the Lord Christ, knowing that they are his true and living members, and that they hunger for this meat of eternal life and thirst for this holy drink. Christian charity, religion, and holy administration have always required this. Other persons, because they cannot participate in the sacrament without condemning themselves, must be kept away from the Holy Supper by the deacon (as the early church commanded). This is true also for those who have not yet been fully instructed in the Christian religion, the wicked, and those who have had to leave the church and who should be making penance, but who have not yet been received in grace. For this reason our Lord himself gave the first supper only to those more elect disciples. It is not fitting to give what is holy to dogs nor to give the meat of eternal life to those who do not hunger for it.
Therefore the Lord's Supper should only be given to those who are known and approved by the rule of charity and religion, a rule which must be practiced in the administration of the Supper, also requiring confession and the acknowledgment that, in our life, nothing contradicts it. Ministers, therefore, act in a holy and correct manner, both by their ministry and by its dignity, when they receive only those persons to the sacrament whom they first of all know to be approved and instructed. Moreover, since this meat and drink of eternal life ought to be administered only to those who truly desire it, it follows that the people to whom the Supper is administered should be admonished so thoroughly as to understand how important it is for them to profit from communion with Christ and what benefits are offered in it to them.