"The Angelic Doctor also notes that the 'effect of this sacrament is the unity of the mystical body [the Church], without which there can be no salvation. Therefore it is necessary to receive the Eucharist, at least by desire (in voto), in order to be saved' (III, q. 73, a. 1, ad 2). These words [...]
"It is written (1 Cor. 11:29): 'He that eateth and drinketh unworthily, eateth and drinketh judgment to himself': and a gloss of the same passage makes the following commentary: 'He eats and drinks unworthily who is in the state of sin, or who handles (the sacrament) irreverently; and such a one eats and drinks judgment, [...]
"In this sacrament we may consider both that from which it derives its effect, namely, Christ contained in it, as also His Passion represented by it; and that through which it works its effect, namely, the use of the sacrament, and its species. Now as to both of these it belongs to this sacrament to [...]
" Just as Christ's Passion has not its effect in them who are not disposed towards it as they should be, so also they do not come to glory through this sacrament who receive it unworthily. Hence Augustine, expounding the same passage, observes: 'The sacrament is one thing, the power of the sacrament another. Many [...]
"The refreshment of spiritual food and the unity denoted by the species of the bread and wine are to be had in the present life, although imperfectly, but perfectly in the state of glory. Hence Augustine says on the words, 'My flesh is meat indeed' (Jn. 6:56)." Source: St. Thomas Aquinas, Summa Theologica [...]
"As Christ's Passion, in virtue whereof this sacrament is accomplished, is indeed the sufficient cause of glory, yet not so that we are thereby forthwith admitted to glory, but we must first 'suffer with Him in order that we may also be glorified' afterwards 'with Him' (Rom. 8:17), so this sacrament does not at once [...]
"Since Christ and His Passion are the cause of grace. and since spiritual refreshment, and charity cannot be without grace, it is clear from all that has been set forth that this sacrament (Eucharist) bestows grace." Source: St. Thomas Aquinas, Summa Theologica
"It is written (Jn. 6:52): 'If any man eat of this bread, he shall live for ever.' But eternal life is the life of glory. Therefore the attaining of glory is an effect of this sacrament." Source: St. Thomas Aquinas, Summa Theologica
"Our Lord says (Jn. 6:52): 'The bread which I will give, is My flesh for the life of the world.' But the spiritual life is the effect of grace. Therefore grace is bestowed through this sacrament... It is considered on the part of what is represented by this sacrament, which is Christ's Passion, as stated [...]
"The effect of this sacrament is considered from the way in which this sacrament is given; for it is given by way of food and drink. And therefore this sacrament does for the spiritual life all that material food does for the bodily life, namely, by sustaining, giving increase, restoring, and giving delight." Source: [...]