Lent is penitential in character. This means that Lent is a time prescribed by the Church to engage in acts of penance and mortification in atonement for the sins we have committed. It is recognized that an integral part of repentance, defined as turning away from sin and back toward God, includes penance both as an expression of sorrow for having offended God and others and as a means of helping to redress the wrongs we have committed.
Fasting has long been recognized as an excellent means of penance, with many spiritual benefits. In the post-modern world, however, the practice of fasting as a means of spiritual benefit has fallen into disuse. The focus is more often on the physical advantages of fasting while its spiritual benefits are disregarded. The Christian recognizes the primary importance of remaining spiritually healthy in view of eternal life, as opposed to a myopic, pagan view in which the material and temporal are given all the emphasis.
What are some of the spiritual benefits of fasting? To explore that question, let's look at the teaching of St. Thomas Aquinas. St. Thomas teaches that fasting is practiced for a threefold purpose: 1) we fast "in order to bridle the lusts of the flesh"; 2) we fast "in order that the mind may arise more freely to the contemplation of heavenly things," noting that Daniel received a revelation from God after fasting for three weeks (Dan 10:2 ff); and 3) we fast "in order to satisfy for sins," as it is written in Joel 2:12: "Be converted to Me with all your heart, in fasting and in weeping and in mourning" (ST, II-II, q. 147, a. 1).
St. Thomas is pointing to some well understood truths in the Catholic spiritual tradition in terms of the spiritual benefits of fasting. First, he notes that fasting helps to bring the body or the flesh under the soul's control. When we fast, we force the body into compliance, which builds self-control and self-mastery, two important virtues in the Christian life. St. Paul speaks about how the desires of the flesh are against the spirit and vice-versa (Gal 5:17). Fasting helps to rectify that disorder, bringing the flesh under the spirit's control, as it should be.
Second, St. Thomas points out that fasting tends to raise the mind and heart to heavenly things, toward the contemplation of God. Fasting empowers us in prayer. It focuses the intellect on seeking to know God and the will on obtaining God as the greatest good. It helps to purify the desires and aspirations of the soul toward the divine beauty and truth of God. Christians who have walked the path of spiritual perfection for some time often report having powerful experiences of the presence of God while fasting. God finds our acts of voluntary suffering for the love of him irresistible.
Third, St. Thomas notes that fasting is a means of atoning for our sins, something we've mentioned already. Fasting is a means of taking responsibility for our sins; it helps us make amends before God for those times we have offended him, others, and his holy Church. This is not to say that fasting confers forgiveness for the guilt of sins. We obtain forgiveness for our sins from God by virtue of the merits of Christ's saving death and resurrection. We are forgiven the eternal guilt of sin through repentance and the sacrament of confession (see John 20:22-23). Nevertheless, we can make restitution before God for our sins through acts of penance like fasting.
Because we are sinners, the Church teaches that Christians are required to do penance. In other words, repentance and penance go hand-in-hand. An example of this is found in the sacrament of Penance. After receiving absolution in the sacrament, penitents are always given some type of penance by the priest who acts as the minister of Christ's forgiveness. Penance, then, is a universal requirement for Christians who commit personal sin.