There are few commandments that bring greater attendant blessings than fasting. Fasting is a sign to the Lord that we are hungering and thirsting for righteousness and the things of his kingdom. It is a powerful element in conforming us to the image of Christ. One of the best scriptures that outlines fasting is Isaiah 58. It explains the kind of fast that the Lord does not want and then outlines a true fast and the blessings that will follow.
The scene opens in Isaiah 58:3 with Israel complaining that God is not giving heed to their fasting. The Lord answers back in verses 3-4, that they have not made their fast different than any other day of the week. In verse 5, the Lord points out that fasting is more than going through the motions of a fast by bowing their heads and sitting in sackcloth and ashes.
The Lord through Isaiah then begins to point out the elements of a true fast:
1. Loose the bands of wickedness (repent) (vs. 6)
2. Undo heavy burdens - giving of yourself spiritually and temporally to others (vs. 6)
3. Let the oppressed go free and break every yoke - kindness and support to those in need (vs. 6)
4. Give food to the hungry (vs. 7)
5. Clothe the naked (vs. 7)
6. Extend your generosity to those of your own family (vs. 7)
The blessing for a true fast are now outlined:
1. Your light will break forth - God's love and blessings are considered light (vs. 8).
2. Your health will increase (vs. 8).
3. You will have a reputation of being righteous (vs. 8).
4. The glory of the Lord will be your rereward or rearguard (vs. 8). Anciently marching armies were often attacked in the rear because that is where the sick and wounded were kept. A rearguard had to be placed there to protect them from the attackers. This appears to be one of the greatest blessings for fasting as the Lord himself promises to guard and protect us where we are the most vulnerable.
5. When you call on the Lord he will answer, if we refrain from being an accusatory type of a person (vs. 9)
6. Your light will rise and be as bright as noonday - no shadow or darkness (vs. 10) Those thus blessed are ready for the presence of God (see D&C 88:67-68)
7. The Lord will guide you continually (vs. 11)
8. Even in a famine you will be fed and watered (vs. 11) - this inference appears to be both physically and spiritually.
9. You will be numbered with those who rebuild Zion and the breaches created by the wickedness of previous generations (vs 12).
The Lord through Isaiah then reminds Israel that keeping the Sabbath holy is also a part of the requirement for receiving the blessings of fasting (vs. 12-14).
President Marion G. Romney gave this counsel:
If we will double our fast offerings, we shall increase our own prosperity, both spiritually and temporally. This the Lord has promised, and this has been recorded. ("Basics of Church Welfare," address to the Priesthood Board, March 6, 1974, p. 10) found in Rulon T. Burton, We Believe [Salt Lake City: Bookcraft, 1994]