I will both lay me down in peace, and sleep - The word “both” here means “at the same time;” that is, I will alike be in peace, and I will lie down and will sleep; I will have a mind at peace (or, in tranquility) when I lie down, and will sleep calmly. This is said in view of his confidence in God, and of his belief that God would preserve him. He had put his trust in him; he had sought his happiness in him, and now he felt assured that he had nothing to fear, and, at peace with God, he would lie down and compose himself to rest. This is the counterpart of what is said in Psalms 3:5. There he says in the morning, that, though surrounded by fear, he “had” been permitted to lie calmly down and sleep; here he says, that, though he is surrounded by fear, he has such confidence in God, that he “will” give himself to quiet slumber. His mind was free from anxiety as to the result of the present troubles; he had calm confidence in God; he committed all to him; and thus gave himself to rest. No one can fail to admire the beauty of this; and no one can fail to perceive that entire confidence in God, and an assurance that all things are under his control, are best adapted of all things to give peaceful days and nights.
For thou, Lord, only makest me dwell in safety - There are two ideas here:
(a) One a confidence that he would abide in safety;
(b) the other, that he owed this entirely to the Lord.
He had no power to defend himself, and yet he felt assured that he would be safe - for he put his trust entirely in the Lord. The whole language implies unwavering trust or confidence in God, and is thus instructive and useful for all. It teaches us:
(1) that in the midst of troubles we may put our trust in God; and
(2) that religion is adapted to make the mind calm in such circumstances, and to enable its possessor to lie down without anxiety in the slumbers of the night, and to pursue without anxiety the duties of the day.