Chapter 33 — Part IV: Prayer — On Prayer in General
Section 6. Besides, so various are our temporal and spiritual necessities, that we must have recourse to prayer as the best means for communicating our wants and receiving whatever we need. For since God owes nothing to anyone, we must ask of Him in prayer those things we need, seeing that He has constituted prayer as a necessary means for the accomplishment of our desires, particularly since it is clear that there are blessings which we cannot hope to obtain otherwise than through prayer. Thus devout prayer has such efficacy that it is a most powerful means of casting out demons; for there is a certain kind of demon which is not cast out but by prayer and fasting.