"Whenever they are attending to the Enemy Himself we are defeated, but there are ways of preventing them from doing so. The simplest is to turn their gaze away from Him towards themselves. Keep them watching their own minds and trying to produce feelings there by the action of their own wills. When they meant to ask Him for charity, let them, instead, start trying to manufacture charitable feelings for themselves and not notice that this is what they are doing. When they meant to pray for courage, let them really be trying to feel brave. When they say they are praying for forgiveness, let them be trying to feel forgiven. Teach them to estimate the value of each prayer by their success in producing the desired feeling; and never let them suspect how much success or failure of that kind depends on whether they are well or ill, fresh or tired, at the moment."
Why how dare that demon Screwtape suggest that anyone's prayers are less than real on the basis of feelings!
How dare he indeed.
(Quote taken from C.S. Lewis' The Screwtape Letters, published by the Macmillan Company, 1943)