Spiritual disciplines are not meritorious works intended to earn God's favor, but rather God-appointed means of grace—such as prayer, fasting, and the study of Scripture—designed to humble the believer, mortify indwelling sin, and foster communion with Christ. They are the 'sinews' of the church that help believers walk in the power of the Holy Spirit, always flowing from the gospel reality that we are already accepted and loved in Christ.
In the Reformed tradition, spiritual disciplines are understood as means of grace. As stated in the 1689 LBCF Ch.22 §5, activities such as the reading of Scripture, prayer, preaching, singing, and solemn humiliation with fasting are essential components of religious worship to be performed in obedience to God. Calvin teaches that these disciplines are not optional; they are the 'sinews' of the church that hold the body together and keep us from lethargy, serving as a fatherly rod to chastise in mercy or a stimulus to arouse the indifferent (Institutes IV.12 §1).
Fasting is a prominent biblical discipline, yet it must be handled with great care to avoid moralism. The Second Helvetic Confession Ch.24 emphasizes that fasting is an 'abstinence and moderation of the godly' intended to humble the flesh so it may more willingly obey the Spirit. It is not a meritorious work. Calvin notes that fasting has three ends: to mortify the flesh, to prepare for prayer, and to evidence our humiliation before God (Institutes IV.12 §15). We must always recall the warning of the Apostle: 'But let every one fast to this end, that he may deprive the flesh of its fuel in order that he may the more zealously serve God.'
The vigor of our spiritual lives relies on the mortification of sin. As John Owen explains, 'the life, vigour, and comfort of our spiritual life depend much on our mortification of sin' (On the Mortification of Sin in Believers Ch.IV). We do not mortify sin to become accepted by God; rather, because we are accepted in Christ, we fight to prune the weeds of lust so that the graces of the Spirit can flourish. We must be wary of moralism, for all true discipline is a fruit of faith—a response to the love that has already redeemed us. As the Apostle writes, 'And they that are of Christ Jesus have crucified the flesh with the passions and the lusts thereof.'