Pope Shenouda III
During the Apostles' fast, the   Holy Spirit spoke to them.
  Thus the Bible says: “As they ministered  to the Lord and fasted, the Holy Spirit said, “Now separate to Me   Barnabas and   Saul  for the work to which I have called them.” Then,  having fasted and  prayed, and laid hands on them, they sent them away.”  (Acts 13:2,3).
  How beautiful then is the Lord’s saying  to the Apostles about fasting and its relation to casting of  devils:
  "However, this kind does not go out except by  prayer and fasting." (Matt 17:21). Such is the power of fasting, in terrorising devils.
  Fasting was not only confined to  individuals, but also for whole nations.