I have proclaimed thy righteousness in the great assembly. Here David again brings forward his own thankfulness, and for no other reason but to induce God to continue his goodness towards him. God, whenever he manifests his liberality towards us, encourages us to render thanks to him; and he continues to act in a similar manner towards us when he sees that we are thankful and mindful of what he has done for us. In the first place, David makes use simply of the word righteousness; but it must be understood of the righteousness of God, which he expressly mentions soon after. Nor does he say, that it was only in the secret affection of the heart, or in private, that he offered praise to God, but that he had openly proclaimed it in the solemn assembly, even as the faithful in those days were wont to testify their devotion by presenting peace-offerings to God when they had been delivered from any great danger. The great assembly of which he speaks is not to be understood of the concourse of people that assemble at courts of law, or at the public market-places, but it denotes the true and lawfully constituted Church of God, which we know assembled in the place of his sanctuary. Accordingly, he declares that he had not concealed in his heart the righteousness of God, which it becomes us publicly to make known for the edification of one another. Those who keep it hid in their hearts are surely seeking as much as in them lies that the memory of God may be buried in oblivion. He calls upon God as a witness of this, not only to distinguish between himself and hypocrites, who often proclaim loudly, and with all their might, the praises of God, and yet do so without the least spark of affection; but also to make it the more abundantly obvious that he had sincerely and heartily uttered the praises of God, and was careful not to defraud him of any part of them. This affirmation teaches us that the subject which is here treated of is one of no small importance; for although God stands in no need of our praises, yet it is his will that this exercise for many reasons should prevail amongst us.

I have preached righteousness in the great congregation - I have main tained and defended the principles of righteousness and truth among assembled multitudes. it would be difficult to see how this could be applied to David himself, or on what occasion of his life this could be said of him; but no one can doubt that this is applicable to the Messiah:
(a) He was a preacher.
(b) He addressed vast multitudes.
(c) Before them all, and at all times, he maintained and illustrated the great principles of "righteousness" as demanded by the law of God, and unfolded the way in which all those multitudes might become righteous before God.
Lo, I have not refrained my lips - I have not closed my lips. I have not kept back the truth.
O Lord, thou knowest - He could make this solemn appeal to God as the Searcher of hearts, in proof that he had faithfully uttered all that had been required of him in making known the will of God. Compare John 17:4, John 17:6,John 17:8, John 17:14, John 17:26.

I have preached righteousness - I think it best to refer these words to Christ and his apostles. In consequence of his having become a sacrifice for sin, the Jewish sacrificial system being ended, the middle wall of partition was broken down, and the door of faith, the doctrine of justification by faith, opened to the Gentiles. Hence the Gospel was preached in all the world, and the mercy of God made known to the Gentiles; and thus righteousness - justification by faith, was preached in the great congregation - to Jews and Gentiles, throughout the Roman empire.
The great congregation, both in this and the following verse, I think, means the Gentiles, contradistinguished from the Jews.
The word righteousness means the plan or method of salvation by Jesus Christ - God's method of justifying sinners by faith, without the deeds of the law. See Romans 3:25-26 (note), and the notes there.

I have preached righteousness in the (h) great congregation: lo, I have not refrained my lips, O LORD, thou knowest.
(h) In the Church assembled in the sanctuary.

I have preached righteousness in the great congregation,.... Not the righteousness which the law requires men to do; but the righteousness which Christ himself wrought out, for the justification of them that believe; this he was a preacher, as well as the author of, and is part of the glad tidings he was anointed to preach, Isaiah 61:1; and the word (n) here used signifies, for the most part, the publishing of good tidings; and this our Lord did publicly, before all the people, in the synagogues of the Jews, and in the temple, whither the people in great numbers resorted; especially at the three great festivals in the year; the feasts of passover, pentecost, and tabernacles, when all the males were obliged to appear, and made up a great congregation indeed; see John 2:23;
lo, I have not refrained my lips, O Lord, thou knowest; Christ appeals to his divine Father, the searcher of hearts, and trier of reins, for the truth of this; that he had not laid any restraint upon his lips, nor kept back anything in his ministry that was profitable; but had taught the way of God in great integrity and sincerity; had opened his mouth, and spoke freely and fully, and used great plainness of speech.
(n) Sept. "evangelizavi", Schmidt, Michaelis; "I have preached the glad tidings of justice", Ainsworth.

I have preached--literally, "announced good tidings." Christ's prophetical office is taught. He "preached" the great truths of God's government of sinners.

Righteousness - Thy faithfulness. Great congregation - In the most public and solemn assemblies: not only to the Jews, but also to all nations; to whom Christ preached by his apostles, as is observed Ephesians 2:17. Not refrained - From preaching it, even to the face of mine enemies.