[The Lord's Prayer], although it seems to contain the utter fullness of perfection inasmuch as it was instituted and established on the authority of the Lord himself, nonetheless raises its familiars to that condition which we characterized previously as more sublime. It leads them by a higher stage to that fiery and, indeed, more properly speaking, wordless prayer which is known and experienced by very few. This transcends all human understanding and is distinguished not, I would say, by a sound of the voice or a movement of the tongue or a pronunciation of words. Rather, the mind is aware of it when it is illuminated by an infusion of heavenly light from it, and not by narrow human words, and once the understanding has been suspended it gushes forth as from a most abundant fountain and speaks ineffably to God, producing more in that very brief moment than the self-conscious mind is able to articulate easily or to reflect upon.
— John Cassian, Conference 9, excerpted
in The Essential Writings of Christian
Mysticism edited by Bernard McGinn