The great danger Paul saw in self-made and self-exalting fasting does not nullify Christian fasting. Paul warns that there is a fasting that is a “self-made religion and [a] self-abasement and severe treatment of the body, [but has] no value against fleshly indulgence” (Colossians 2:23). In other words, this fasting is a “willpower religion” that actually stirs up the spiritual pride of the flesh even while mastering its physical appetites.

But this is the exact opposite of Christian fasting. Christian fasting moves from broken and contrite poverty of spirit to sweet satisfaction in the free mercy of Christ to ever greater desires and enjoyments of God’s inexhaustible grace. Christian fasting does not bolster pride, because it rests with childlike contentment in the firmly accomplished justification of God in Christ, even while longing for all the fullness of God possible in this life. Christian fasting is the effect of what Christ has already done for us and in us. It is not our feat, but the Spirit’s fruit. Recall that the last-mentioned fruit of the Spirit is “self-control” (Galatians 5:23).

John Piper, Hunger for God: Desiring God Through Fasting and Prayer, p. 45