Reply Obj. 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 (Q. 73, A. 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 Obj. 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 (Isa. 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 Cor. 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 Cant 5:1: "Eat, O friends, and drink, and be inebriated, my dearly beloved."
Reply Obj. 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 (Q. 76, A. 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 [Vulg.: 'your'] members" as "instruments of justice unto God" (Rom. 6:13), and in the life to come our body will share in the incorruption and the glory of the soul. _______________________
SECOND ARTICLE [III, Q. 79, Art. 2]
Whether the Attaining of Glory Is an Effect of This Sacrament?
Objection 1: It seems that the attaining of glory is not an effect of this sacrament. For an effect is proportioned to its cause. But this sacrament belongs to "wayfarers" (viatoribus), and hence it is termed "Viaticum." Since, then, wayfarers are not yet capable of glory, it seems that this sacrament does not cause the attaining of glory.
Obj. 2: Further, given sufficient cause, the effect follows. But many take this sacrament who will never come to glory, as Augustine declares (De Civ. Dei xxi). Consequently, this sacrament is not the cause of attaining unto glory.
Obj. 3: Further, the greater is not brought about by the lesser, for nothing acts outside its species. But it is the lesser thing to receive Christ under a strange species, which happens in this sacrament, than to enjoy Him in His own species, which belongs to glory. Therefore this sacrament does not cause the attaining of glory.
On the contrary, It is written (John 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.
I answer that, 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.