I will sing of the mercies of the Lord for ever - Particularly how the “mercy” was manifested in the promise made to David; the solemn covenant made with him in respect to the perpetuity of his throne. The appointment of David to the throne was an act of mere mercy or favor, since he was not in the royal line, and had no claim to the crown. It will be seen, also, that if it be supposed that the covenant with David, and the promise therein made to him, was intended to include the Messiah as descending from him, there was a still higher reason for celebrating the “mercies” of God, inasmuch as all mercy to our world comes through him.
With my mouth - Not merely in my heart, but with words. The meaning here is that he would make a record which might be used evermore as the language of praise.
Will I make known thy faithfulness - In the fulfillment of these promises. He felt assured that they would be fulfilled. Whatever appearances there might be to the contrary, the psalmist had no doubt that God would prove himself to be faithful and true. See the notes at Isaiah 55:3, on the expression, “the sure mercies of David.”
To all generations - Margin, as in Hebrew, generation and generation. He would make a record which would carry down the remembrance of this faithfulness to all future ages.