Last of all, David confesses that it was entirely owing to the pure grace of God that he had come to possess so great a good, and that he had been made a partaker of it by faith. It would be of no advantage to us for God to offer himself freely and graciously to us, if we did not receive him by faith, seeing he invites to himself both the reprobate and the elect in common; but the former, by their ingratitude, defraud themselves of this inestimable blessing. Let us, therefore, know that both these things proceed from the free liberality of God; first, his being our inheritance, and next, our coming to the possession of him by faith. The counsel of which David makes mention is the inward illumination of the Holy Spirit, by which we are prevented from rejecting the salvation to which he calls us, which we would otherwise certainly do, considering the blindness of our flesh. Whence we gather, that those who attribute to the free will of man the choice of accepting or rejecting the grace of God basely mangle that grace, and show as much ignorance as impiety. That this discourse of David ought not to be understood of external teaching appears clearly from the words, for he tells us that he was instructed in the night when he was removed from the sight of men. Again, when he speaks of this being done in his reins, he doubtless means secret inspirations. [1] Farther, it ought to be carefully observed, that, in speaking of the time when he was instructed, he uses the plural number, saying, it was done in the nights. By this manner of speaking, he not only ascribes to God the beginning of faith, but acknowledges that he is continually making progress under his tuition; and, indeed, it is necessary for God, during the whole of our life, to continue to correct the vanity of our minds, to kindle the light of faith into a brighter flame, and by every means to advance us higher in the attainments of spiritual wisdom.

Footnotes

1 - Calvin means that God taught David by secret inspirations.

I will bless the Lord, who hath given the counsel - Probably the reference here is to the fact that the Lord had counseled him to choose him as his portion, or had inclined him to his service. There is nothing for which a heart rightly affected is more disposed to praise God than for the fact that by his grace it has been inclined to serve him; and the time when the heart was given away to God is recalled ever onward as the happiest period of life.
My reins - See the notes at Psalm 7:9. The "reins" are here put for the mind, the soul. They were regarded as the seat of the affections, Jeremiah 11:20; Job 19:27. The meaning here is, that in the wakeful hours of night, when meditating on the divine character and goodness, he found instruction in regard to God. Compare Psalm 17:3. Everything then is favorable for reflection. The natural calmness and composure of the mind; the stillness of night; the starry heavens; the consciousness that we are alone with God, and that no human eye is upon us - all these things are favorable to profound religious meditation. They who are kept wakeful by night "need" not find this an unprofitable portion of their lives. Some of the most instructive hours of life are those which are spent when the eyes refuse to close themselves in slumber, and when the universal stillness invites to contemplation on divine things.

Who hath given me counsel - Jesus, as man, received all his knowledge and wisdom from God; Luke 2:40-52. And in him were hidden all the treasures of wisdom and knowledge.
My reins also instruct me - כליותי kilyothai, reins or kidneys, which from their retired situation in the body, says Parkhurst, and being hidden in fat, are often used in Scripture for the most secret workings and affections of the heart.
The kidneys and their fat were always to be burnt in sacrifice, to indicate that the most secret purposes and affections of the soul are to be devoted to God.
In the night seasons - That is, in the time of my passion, my secret purposes and determinations concerning the redemption of man support me. "For the joy that was set before him he endured the cross, despising the shame;" Hebrews 12:2.

I will bless the Lord,.... As prayer, so thanksgiving belongs to Christ, as man and Mediator; see Matthew 11:25; and here he determines to praise the Lord, and give thanks to him for counsel and instruction:
who hath given me counsel; for though he himself is the Counsellor, with respect to his people, yet as man he received counsel from God, and the spirit of counsel rested on him, Isaiah 11:2; and fitted him for and directed him in the execution of his prophetic office; for the doctrine he taught was not his own, but his Father's; and he said nothing of himself but what his Father taught him, and instructed him to speak, John 6:16. And he also gave him counsel about the execution of his priestly office, or about his sufferings and death, and drinking of the cup, which he, with submission to the divine will, desired might pass from him; but having advice in this matter, most cheerfully and courageously yielded to take it, see Matthew 26:39;
my reins also instruct me in the night seasons; when engaged in prayer to God, in which he sometimes continued a whole night together, Luke 6:12; and especially in that dark and dismal night in which he was betrayed, when it was the hour and power of darkness with his enemies; then, his inward parts being influenced by the spirit of wisdom and counsel, directed him how to behave and conduct himself. Or "the reins" being the seat of the affections, and being put for them, may signify, that his strong affection for God, and love to his people, put him upon and moved him to take the steps he did, to deliver up himself into the hands of sinful men, in order to suffer and die for his friends, and obtain eternal salvation for them.

given me counsel--cared for me.
my reins--the supposed seat of emotion and thought (Psalm 7:9; Psalm 26:2).
instruct me--or, excite to acts of praise (Isaiah 53:11-12; Hebrews 12:2).

The Lord - Hath inspired that wisdom into me, by which I have chosen the Lord for my portion, and am so fully satisfied with him. Reins - My inward thoughts and affections, being inspired and moved by the holy spirit. Instruct - Direct me how to please God, and put my whole trust in him. Night - Even when others are asleep, my mind is working upon God, and improving the silence and solitude of holy meditations.