... and humbled myself with fasting. —Psalm 35:13
You might be thinking, I'm willing to fast, but when do I do it? I would therefore like to suggest five occasions on which fasting is justifiable.
I think the first of those would be when the burden we are under is so great that we do not really have a desire for food—for this may be a hint from God that we should fast. David experienced a time of great mourning when God smote the son born to him as a result of his adultery with Bathsheba (2 Sam. 12:15-16).
The second occasion that justifies fasting is when we are about to embark on a very great task for God or have to make an important decision. At the beginning of His ministry, after His baptism, Jesus fasted (Matt. 4:2). Maybe you need to know God's will and don't know what to do. It is justifiable to fast because you need wisdom for something in the future.
A third reason is if we feel that God is hiding His face. Perhaps we are in a rut or have known better days spiritually. Perhaps God is not as real as we have known Him to be, and we are not sure whether we have grieved Him or whether He has just chosen to hide His face for reasons we can't understand. Perhaps God is hiding His face from us in order to drive us to our knees to seek Him.
The fourth occasion is when we have experienced delay in the answers to our prayers. In the Old Testament in particular we have accounts of situations where God did not step in as it had been hoped He would, and as a result the people fasted.
The fifth occasion that justifies fasting is when we feel the need of unusual power that we don't have, such as in the case of demon possession in Matthew 17, which was too big for the disciples to handle.
Fasting is a way of ensuring that we are completely dependent upon God and open to Him. It seeks spiritual emptiness and cleansing, and it enables us to hear God speaking.
Excerpted from Worshipping God (Hodder & Stoughton, 2004).