The devil employs all his efforts to hinder prayer or to make it powerless and ineffective. For this spirit–cast down from heaven for pride and rebellion against God, infected with incurable envy and hatred for the human race, burning with thirst for the destruction of men, sleeplessly engaged day and night in man’s ruin–it is intolerable to see weak and sinful man detach himself by prayer from everything earthly, and enter into conversation with God Himself and go out from this conversation sealed with the mercy of God, in hope of inheriting heaven and seeing even his frail body transformed into a spiritual body.
This spectacle is unbearable for a spirit who is forever condemned to creep and crawl, as in mud and stench, in thoughts and feelings exclusively carnal, material, sinful, and who must finally be cast down and confined for all eternity in the prisons of hell.
~Ignatius Brianchaninov
Prayerful Life 