Council of Trent: Session 13
Chapter 7
On the preparation to be given that one may worthily receive the sacred Eucharist.
If it is unbeseeming for any one to approach to any of the sacred functions, unless he approach holily; assuredly, the more the holiness and divinity of this heavenly sacrament are understood by a Christian, the more diligently ought he to give heed that he approach not to receive it but with great reverence and holiness, especially as we read in the Apostle those words full of terror; He that eateth and drinketh unworthily, eateth and drinketh judgment to himself. Wherefore, he who would communicate, ought to recall to mind the precept of the Apostle; Let a man prove himself. Now ecclesiastical usage declares that necessary proof to be, that no one, conscious to himself of mortal sin, how contrite soever he may seem to himself, ought to approach to the sacred Eucharist without previous sacramental confession. This the holy Synod hath decreed is to be invariably observed by all Christians, even by those priests on whom it may be incumbent by their office to celebrate, provided the opportunity of a confessor do not fail them; but if, in an urgent necessity, a priest should celebrate without previous confession, let him confess as soon as possible.
Chapter 8
On the use of this admirable Sacrament.
Now as to the use of this holy sacrament, our Fathers have rightly and wisely distinguished three ways of receiving it. For they have taught that some receive it sacramentally only, to wit sinners: others spiritually only, those to wit who eating in desire that heavenly bread which is set before them, are, by a lively faith which worketh by charity, made sensible of the fruit and usefulness thereof: whereas the third (class) receive it both sacramentally and spiritually, and these are they who so prove and prepare themselves beforehand, as to approach to this divine table clothed with the wedding garment. Now as to the reception of the sacrament, it was always the custom in the Church of God, that laymen should receive the communion from priests; but that priests when celebrating should communicate themselves; which custom, as coming down from an apostolical tradition, ought with justice and reason to be retained. And finally this holy Synod with true fatherly affection admonishes, exhorts, begs, and beseeches, through the bowels of the mercy of our God, that all and each of those who bear the Christian name would now at length agree and be of one mind in this sign of unity, in this bond of charity, in this symbol of concord; and that mindful of the so great majesty, and the so exceeding love of our Lord Jesus Christ, who gave His own beloved soul as the price of our salvation, and gave us His own flesh to eat, they would believe and venerate these sacred mysteries of His body and blood with such constancy and firmness of faith, with such devotion of soul, with such piety and worship as to be able frequently to receive that supersubstantial bread, and that it may be to them truly the life of the soul, and the perpetual health of their mind; that being invigorated by the strength thereof, they may, after the journeying of this miserable pilgrimage, be able to arrive at their heavenly country, there to eat, without any veil, that same bread of angels which they now eat under the sacred veils.
But forasmuch as it is not enough to declare the truth, if errors be not laid bare and repudiated, it hath seemed good to the holy Synod to subjoin these canons, that all, -the Catholic doctrine being already recognised,-may now also understand what are the heresies which they ought to guard against and avoid.