Abba Agathon was asked, "Amongst all our different activities, father, which is the virtue that requires the greatest effort?" Agathon said, "Forgive me, but I think there is no labor greater than praying to God. For every time a man wants to pray, his enemies the demons try to prevent him; for they know that nothing obstructs them so much as prayer to God. In everything else a man undertakes, if he persevers, he will attain rest. But in order to pray a man must struggle to his last breath" (The Sayings of the Desert Fathers).
Wrote Screwtape to his nephew Wormwood: "It is, no doubt, impossible to prevent his praying for his mother, but we have means of rendering the prayers innocuous. Make sure that they are always very 'spiritual', that he is always concerned with the state of her soul and never with her rheumatism. Two advantages follow. In the first place, his attention will be kept on what he regards as her sins, by which, with a little guidance from you, he can be induced to mean any of her actions which are inconvenient or irritating to himself. Thus you can keep rubbing the wounds of the day a little sorer even while he is on his knees; the operation is not at all difficult and you will find it very entertaining. In the second place, since his ideas about her soul will be very crude and often erroneous, he will, in some degree, be praying for an imaginary person, and it will be your task to make that imaginary person daily less and less like the real mother—the sharp-tongued old lady at the breakfast table. In time, you may get the cleavage so wide that no thought or feeling from his prayers for the imagined mother will ever flow over into his treatment of the real one. I have had patients of my own so well in hand that they could be turned at a moment's notice from impassioned prayer for a wife's or son's 'soul' to beating or insulting the real wife or son without a qualm" (C.S. Lewis The Screwtape Letters).