Fasting for Spiritual Strength

Joel 2:11-13, 21-25 King James Version (KJV)

11 And the Lord shall utter his voice before his army: for his camp is very great: for he is strong that executeth his word: for the day of the Lordis great and very terrible; and who can abide it?

12 Therefore also now, saith the Lord, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning:

13 And rend your heart, and not your garments, and turn unto the Lordyour God: for he is gracious and merciful, slow to anger, and of great kindness, and repenteth him of the evil. 

God Promnises:

21 Fear not, O land; be glad and rejoice: for the Lord will do great things.

22 Be not afraid, ye beasts of the field: for the pastures of the wilderness do spring, for the tree beareth her fruit, the fig tree and the vine do yield their strength.

23 Be glad then, ye children of Zion, and rejoice in the Lord your God: for he hath given you the former rain moderately, and he will cause to come down for you the rain, the former rain, and the latter rain in the first month.

24 And the floors shall be full of wheat, and the vats shall overflow with wine and oil.

25 And I will restore to you the years that the locust hath eaten, the cankerworm, and the caterpiller, and the palmerworm, my great army which I sent among you.