For this verse has not unreasonably been picked out from the whole of Scripture for this purpose.
For it embraces all the feelings which can be implanted in human nature, and can be fitly and satisfactorily adapted to every condition, and all assaults.
Since it contains an invocation of God against every danger, it contains humble and pious confession, it contains the watchfulness of anxiety and continual fear, it contains the thought of one’s own weakness, confidence in the answer, and the assurance of a present and ever ready help.
For one who is constantly calling on his protector, is certain that He is always at hand.
[…] This thought in your heart may be to you a saving formula, and not only keep you unharmed by all attacks of devils, but also purify you from all faults and earthly stains, and lead you to that invisible and celestial contemplation, and carry you on to that ineffable glow of prayer, of which so few have any experience.
Let sleep come upon you still considering this verse, till having been moulded by the constant use of it, you grow accustomed to repeat it even in your sleep.
When you wake let it be the first thing to come into your mind, let it anticipate all your waking thoughts, let it when you rise from your bed send you down on your knees, and thence send you forth to all your work and business, and let it follow you about all day long.
This you should think about, according to the Lawgiver’s charge, “at home and walking forth on a journey” (Deut. 6:7), sleeping and waking.
This you should write on the threshold and door of your mouth, this you should place on the walls of your house and in the recesses of your heart so that when you fall on your knees in prayer this may be your chant as you kneel, and when you rise up from it to go forth to all the necessary business of life it may be your constant prayer as you stand.
John Cassian (c. 360-435): Conferences 10, 10.