" Here the evil spirit hinders men with all his powers. Oh, how often will he here prevent the desire to pray, not allow us to find time and place, nay, often also raise doubts, whether a man is worthy to ask anything of such a Majesty as God is, and so confuse us that a man himself does not know whether it is really true that he prays or not; whether it is possible that his prayer is acceptable, and other such strange thoughts. For the evil spirit knows well how powerful one man's truly believing prayer is, and how it hurts him, "

― Martin Luther , A Treatise on Good Works