"The sin of unbelief, which fundamentally severs a man from the unity of the Church, simply speaking, makes him to be utterly unfit for receiving this sacrament; because it is the sacrament of the Church's unity, as stated above. Hence the unbeliever who receives this sacrament sins more grievously than the believer who is in [...]
"Every medicine does not suit every stage of sickness; because the tonic given to those who are recovering from fever would be hurtful to them if given while yet in their feverish condition. So likewise Baptism and Penance are as purgative medicines, given to take away the fever of sin; whereas this sacrament (Eucharist) is [...]
"This sacrament is not only a sacrament, but also a sacrifice. For, it has the nature of a sacrifice inasmuch as in this sacrament Christ's Passion is represented, whereby Christ 'offered Himself a Victim to God' (Eph. 5:2), and it has the nature of a sacrament inasmuch as invisible grace is bestowed in this sacrament [...]
"The effect of this sacrament (Eucharist) 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, [...]
"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 Ps. 103:5: '(That) bread strengthens [Vulg.: 'may strengthen'] man's heart.' Augustine likewise says (Tract. xxvi in Joan.): 'Approach without [...]
"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 [...]
"It (Eucharist) was instituted not for satisfaction, but for nourishing spiritually through union between Christ and His members, as nourishment is united with the person nourished. But because this union is the effect of charity, from the fervor of which man obtains forgiveness, not only of guilt but also of punishment, hence it is that [...]
"Two things may be considered in this sacrament, to wit, the sacrament itself, and the reality of the sacrament: and it appears from both that this sacrament has the power of forgiving venial sins. For this sacrament is received under the form of nourishing food. Now nourishment from food is requisite for the body to [...]
"The reality of this sacrament is charity, not only as to its habit, but also as to its act, which is kindled in this sacrament; and by this means venial sins are forgiven. Consequently, it is manifest that venial sins are forgiven by the power of this sacrament." Source: St. Thomas Aquinas, Summa Theologica [...]
"The power of this sacrament can be considered in two ways. First of all, in itself: and thus this sacrament has from Christ's Passion the power of forgiving all sins, since the Passion is the fount and cause of the forgiveness of sins. Secondly, it can be considered in comparison with the recipient of the [...]