Luther

“Let them undertake just prayer alone and rightly exercise themselves in faith, and they will find that it is true, as the holy fathers have said, that there is no work like prayer. Mumbling with the mouth is easy, or at least considered easy. But to follow the words in deep devotion and sincerity of heart, that is, with desire and in faith, so that one earnestly desires what the words say and does not doubt that that they will be heard, that is a great deed in God's sight.” Martin Luther, Treatise on Good Works, LW, AE, 44:61-2