"We are aware of the torment that the enemy frequently inflicts upon us visibly or invisibly. But we do not perceive the torment and anguish that we inflict upon him, when we sometimes succeed in practising [sic], when we reprent over our transgressions or show long-suffering and perserverance in our difficulties, or when we pray and do other things which piece him to the heart, torture him and cause him bitter grief. God in His providence conceals all this from us, so as to prevent us from growing sluggish. Be sure, however, that 'God thinks it right to repay with affliction those who afflict you' (2 Thessalonians 1:6)."
(St. John of Karpathos, an excerpt from Texts for the Monks in India in The Philokalia, Volume 1, p. 316.)