Pope Shenouda III
 
In all this, be ready to strive  against the   Devil.
Joshua, son of Sirakh, told his son that  if he set forward to serve God, he should prepare himself for all  trials.
When the   Devil  sees your fast and  repentance, he becomes envious of your spiritual  acts. Thus, he fights  you to deprive you of the fruit of your labour  and seeks every trick to  bring about your downfall, saying: I shall not  leave you until you give  up. Remember the words of the  Apostle Peter, who said,  “Resist him, steadfast in the faith. “ (1 Pet 5:9).
Thus, fasting  is a period of spiritual  warfare as was the case with  Jesus  Christ.  (Matt 4). It is also a period  of triumph for him who shares it with   Christ.