Since prayer is an association of the mind with God, then in what state must the mind possibly be, in order to be able, without turning elsewhere, to approach its Lord and converse with Him without the mediation of something else? If Moses in his attempt to approach the burning bush was hindered until he had removed the sandals from his feet, then shouldn’t you who desires to see God and converse with Him, remove and cast out of you every sinful thought?
–Saint Nilus of Sinai (d. 430)