Saint Thomas Aquinas, in Summa Theologica, writing in response to the gospel reading today for John on the “bread of life,” says 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, man’s nature is strengthened inwardly against inner decay, and so by means of food and medicine, he is preserved from death. Secondly, he is guarded against outward assaults, and thus, he is protected by the means of arms by which he defends his body. Now, the Eucharist 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, “That bread strengthens strengthen man’s heart (Ps. 103:5).” Augustine likewise says, “Approach without fear. It is bread, not poison.” Secondly, because it is a sign of Christ’s Passion, whereby the devils are conquered, it repels all the assaults of demons. Hence, Chrysostom says, “Like lions breathing forth fire, thus do we depart from that table, being made terrible to the devil.” Indeed, it is also true that many, after receiving this sacrament, worthily fall again into sin, but it is due to the changeableness of free will that man sins after possessing charity, for his free will can easily be fixed on good or evil. Hence, although this sacrament itself has the power to preserve us from sin, it does not take away from man the possibility of sinning. And the same must be said of charity. For charity in itself preserves man from sin, but because of the weakness of free will, it happens that one sins after possessing charity just as one does after receiving this sacrament. Although this sacrament is ordained directly to lessen the inclination to sin, it does lessen it as a consequence, inasmuch as it increases charity because, as Augustine says, “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.