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.
Objection 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.
Objection 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.
Now as to both of these it belongs to this sacrament to cause the attaining of eternal life. Because it was by His Passion that Christ opened to us the approach to eternal life, according to Hebrews 9:15: "He is the Mediator of the New Testament; that by means of His death... they that are called may receive the promise of eternal inheritance." Accordingly in the form of this sacrament it is said: "This is the chalice of My blood, of the New and Eternal Testament."
In like manner 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" (John 6:56): "Seeing that in meat and drink, men aim at this, that they hunger not nor thirst, this verily nought doth afford save only this meat and drink which maketh them who partake thereof to be immortal and incorruptible, in the fellowship of the saints, where shall be peace, and unity, full and perfect."
Reply to Objection 1. 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" (Romans 8:17), so this sacrament does not at once admit us to glory, but bestows on us the power of coming unto glory. And therefore it is called "Viaticum," a figure whereof we read in 1 Kings 19:8: "Elias ate and drank, and walked in the strength of that food forty days and forty nights unto the mount of God, Horeb."
Reply to Objection 2. 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 (Tract. xxvi in Joan.), expounding the same passage, observes: "The sacrament is one thing, the power of the sacrament another. Many receive it from the altar... and by receiving"... die... Eat, then, spiritually the heavenly "bread, bring innocence to the altar." It is no wonder, then, if those who do not keep innocence, do not secure the effect of this sacrament.
Reply to Objection 3. That Christ is received under another species belongs to the nature of a sacrament, which acts instrumentally. But there is nothing to prevent an instrumental cause from producing a more mighty effect, as is evident from what was said above (77, 3, ad 3).
Source: Summa Theologica (New Advent)