St. Thomas Aquinas
“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 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.'”
Source: St. Thomas Aquinas, Summa Theologica