John Calvin Commentary Psalms 40:9

"I have proclaimed glad tidings of righteousness in the great assembly; Lo, I will not refrain my lips, O Jehovah, thou knowest." — Psalms 40:9 (ASV)

I have proclaimed your righteousness in the great assembly. Here David again brings forward his own thankfulness, for no other reason than to induce God to continue His goodness toward him. God, whenever He shows His generosity toward us, encourages us to give thanks to Him; and He continues to act in a similar manner toward us when He sees that we are thankful and mindful of what He has done for us.

In the first place, David simply uses the word righteousness; but it must be understood as 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, just as the faithful in those days were accustomed 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 as the gathering of people that assemble at courts of law or at public marketplaces, but it denotes the true and lawfully constituted Church of God, which, as 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 is fitting for us to publicly make known for our mutual edification.

Those who keep it hidden in their hearts are surely seeking, as much as they can, for the memory of God to be lost to oblivion. He calls upon God as a witness of this, not only to distinguish himself from hypocrites, who often proclaim loudly and with all their strength the praises of God, and yet do so without the least spark of affection; but also to make it abundantly clear that he had sincerely and heartily uttered the praises of God and was careful not to deprive Him of any part of them.

This affirmation teaches us that the subject discussed here is of considerable importance; for although God does not need our praises, it is His will that this practice, for many reasons, should be widespread among us.