Objection 1. It seems that grace is not bestowed through this sacrament. For this sacrament is spiritual nourishment. But nourishment is only given to the living. Therefore since the spiritual life is the effect of grace, this sacrament belongs only to one in the state of grace. Therefore grace is not bestowed through this sacrament for it to be had in the first instance. In like manner neither is it given so as grace may be increased, because spiritual growth belongs to the sacrament of Confirmation, as stated above (Question 72, Article 1). Consequently, grace is not bestowed through this sacrament.
Objection 2. Further, this sacrament is given as a spiritual refreshment. But spiritual refreshment seems to belong to the use of grace rather than to its bestowal. Therefore it seems that grace is not given through this sacrament.
Objection 3. Further, as was said above (Question 74, Article 1), "Christ's body is offered up in this sacrament for the salvation of the body, and His blood for that of the soul." Now it is not the body which is the subject of grace, but the soul, as was shown in I-II, 110, 4. Therefore grace is not bestowed through this sacrament, at least so far as the body is concerned.
On the contrary, Our Lord says (John 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.
I answer that, The effect of this sacrament ought to be considered, first of all and principally, from what is contained in this sacrament, which is Christ; Who, just as by coming into the world, He visibly bestowed the life of grace upon the world, according to John 1:17: "Grace and truth came by Jesus Christ," so also, by coming sacramentally into man causes the life of grace, according to John 6:58: "He that eateth Me, the same also shall live by Me." Hence Cyril says on Luke 22:19: "God's life-giving Word by uniting Himself with His own flesh, made it to be productive of life. For it was becoming that He should be united somehow with bodies through His sacred flesh and precious blood, which we receive in a life-giving blessing in the bread and wine."
Secondly, it is considered on the part of what is represented by this sacrament, which is Christ's Passion, as stated above (74, 1; 76, 2, ad 1). And therefore this sacrament works in man the effect which Christ's Passion wrought in the world. Hence, Chrysostom says on the words, "Immediately there came out blood and water" (John 19:34): "Since the sacred mysteries derive their origin from thence, when you draw nigh to the awe-inspiring chalice, so approach as if you were going to drink from Christ's own side." Hence our Lord Himself says (Matthew 26:28): "This is My blood... which shall be shed for many unto the remission of sins."
Thirdly, 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. Accordingly, Ambrose says (De Sacram. v): "This is the bread of everlasting life, which supports the substance of our soul." And Chrysostom says (Hom. xlvi in Joan.): "When we desire it, He lets us feel Him, and eat Him, and embrace Him." And hence our Lord says (John 6:56): "My flesh is meat indeed, and My blood is drink indeed."
Fourthly, the effect of this sacrament is considered from the species under which it is given. Hence Augustine says (Tract. xxvi in Joan.): "Our Lord betokened His body and blood in things which out of many units are made into some one whole: for out of many grains is one thing made," viz. bread; "and many grapes flow into one thing," viz. wine. And therefore he observes elsewhere (Tract. xxvi in Joan.): "O sacrament of piety, O sign of unity, O bond of charity!"
And 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 bestows grace.
Reply to Objection 1. This sacrament has of itself the power of bestowing grace; nor does anyone possess grace before receiving this sacrament except from some desire thereof; from his own desire, as in the case of the adult. or from the Church's desire in the case of children, as stated above (Question 73, Article 3). Hence it is due to the efficacy of its power, that even from desire thereof a man procures grace whereby he is enabled to lead the spiritual life. It remains, then, that when the sacrament itself is really received, grace is increased, and the spiritual life perfected: yet in different fashion from the sacrament of Confirmation, in which grace is increased and perfected for resisting the outward assaults of Christ's enemies. But by this sacrament grace receives increase, and the spiritual life is perfected, so that man may stand perfect in himself by union with God.
Reply to Objection 2. This sacrament confers grace spiritually together with the virtue of charity. Hence Damascene (De Fide Orth. iv) compares this sacrament to the burning coal which Isaias saw (Isaiah 6:6): "For a live ember is not simply wood, but wood united to fire; so also the bread of communion is not simple bread but bread united with the Godhead." But as Gregory observes in a Homily for Pentecost, "God's love is never idle; for, wherever it is it does great works." And consequently through this sacrament, as far as its power is concerned, not only is the habit of grace and of virtue bestowed, but it is furthermore aroused to act, according to 2 Corinthians 5:14: "The charity of Christ presseth us." Hence it is that the soul is spiritually nourished through the power of this sacrament, by being spiritually gladdened, and as it were inebriated with the sweetness of the Divine goodness, according to Canticles 5:1: "Eat, O friends, and drink, and be inebriated, my dearly beloved."
Reply to Objection 3. Because the sacraments operate according to the similitude by which they signify, therefore by way of assimilation it is said that in this sacrament "the body is offered for the salvation of the body, and the blood for the salvation of the soul," although each works for the salvation of both, since the entire Christ is under each, as stated above (Question 76, Article 2). And although the body is not the immediate subject of grace, still the effect of grace flows into the body while in the present life we present "our [Vulgate: 'your'] members" as "instruments of justice unto God" (Romans 6:13), and in the life to come our body will share in the incorruption and the glory of the soul.
Source: Summa Theologica (New Advent)