An excerpt from my notes on the Day of Atonement
On the day of Atonement, the Israelites were to practice "self-denial" (traditionally understood to mean fasting) (Lev. 23:32). Isaiah 58:6 and 7 say,
Is not this the kind of fasting I have chosen: to loose the chains of injustice and untie the cords of the yoke, to set the oppressed free and break every yoke? Is it not to share your food with the hungry and to provide the poor wanderer with shelter--when you see the naked, to clothe him, and not to turn away from your own flesh and blood?
These verses show that God values a self-giving lifestyle much more than ritual self-deprivation. God wants us to live from the heart, not the stomach. This is why Jesus said, “If anyone wants to come after me, he must deny himself, take up his cross daily, and follow me” (Luke 9:23). The Christ lifestyle is one lived unto the cross. This is true fasting.Second Corinthians 5:15 says, “And he died for all, that those who live should no longer live for themselves but for him who died for them and was raised again.” For any Israelite who took to heart the full significance of the sin offering on the Day of Atonement, not eating might be a natural response of repentance, besides being commanded by God. In the same way, we respond to Christ's sin offering by "fasting," by living for Him and not ourselves. This isn't a rule God imposes on us. It is the natural response of a heart changed by the love of Christ who gave Himself for us.