Objection 1. It seems that man is not preserved by this sacrament from future sins. For there are many that receive this sacrament worthily, who afterwards fall into sin. Now this would not happen if this sacrament were to preserve them from future sins. Consequently, it is not an effect of this sacrament to preserve from future sins.
Objection 2. Further, the Eucharist is the sacrament of charity, as stated above (Article 4). But charity does not seem to preserve from future sins, because it can be lost through sin after one has possessed it, as was stated in the II-II, 24, 11. Therefore it seems that this sacrament does not preserve man from sin.
Objection 3. Further, the origin of sin within us is "the law of sin, which is in our members," as declared by the Apostle (Romans 7:23). But the lessening of the fomes, which is the law of sin, is set down as an effect not of this sacrament, but rather of Baptism. Therefore preservation from sin is not an effect of this sacrament.
On the contrary, our Lord said (John 6:50): "This is the bread which cometh down from heaven; that if any man eat of it, he may not die": which manifestly is not to be understood of the death of the body. Therefore it is to be understood that this sacrament preserves from spiritual death, which is through sin.
I answer that, Sin is the spiritual death of the soul. Hence man is preserved from future sin in the same way as the body is preserved from future death of the body: and this happens in two ways. First of all, in so far as man's nature is strengthened inwardly against inner decay, and so by means of food and medicine he is preserved from death. Secondly, by being guarded against outward assaults; and thus he is protected by means of arms by which he defends his body.
Now this sacrament preserves man from sin in both of these ways. For, first of all, by uniting man with Christ through grace, it strengthens his spiritual life, as spiritual food and spiritual medicine, according to Psalm 103:5: "(That) bread strengthens [Vulgate: 'may strengthen'] man's heart." Augustine likewise says (Tract. xxvi in Joan.): "Approach without fear; it is bread, not poison." Secondly, inasmuch as it is a sign of Christ's Passion, whereby the devils are conquered, it repels all the assaults of demons. Hence Chrysostom says (Hom. xlvi in Joan.): "Like lions breathing forth fire, thus do we depart from that table, being made terrible to the devil."
Reply to Objection 1. The effect of this sacrament is received according to man's condition: such is the case with every active cause in that its effect is received in matter according to the condition of the matter. But such is the condition of man on earth that his free-will can be bent to good or evil. Hence, although this sacrament of itself has the power of preserving from sin, yet it does not take away from man the possibility of sinning.
Reply to Objection 2. Even charity of itself keeps man from sin, according to Romans 13:10: "The love of our neighbor worketh no evil": but it is due to the mutability of free-will that a man sins after possessing charity, just as after receiving this sacrament.
Reply to Objection 3. Although this sacrament is not ordained directly to lessen the fomes, yet it does lessen it as a consequence, inasmuch as it increases charity, because, as Augustine says (83), "the increase of charity is the lessening of concupiscence." But it directly strengthens man's heart in good; whereby he is also preserved from sin.
Source: Summa Theologica (New Advent)