In God we will boast [1] all the day This is the conclusion of the first part of the psalm. To express the meaning in a few words, they acknowledge, that in all ages the goodness of God had been so great towards the children of Abraham, that it furnished them with continual matter of thanksgiving. As if the thing were still present to their view, they acknowledge that, without ceasing, they ought to give praise to God, because they had flourished and triumphed, not merely for one age, or a short period of time, but because they had continued to do so successively from age to age, for whatever prosperity had befallen them, they ascribe it to the grace of God. And, certainly, it is then that men experience from the prosperity which befalls them, a holy and a well-regulated joy, when it bursts forth in the praises of God. Let us then, in the first place, bear in mind that this verse relates to the time of joy and prosperity in which God manifested his favor towards his people; secondly, that the faithful here manifest that they are not ungrateful, inasmuch as, having laid aside all vain boasting, they confess that all the victories by which they had become great and renowned proceeded from God, and that it was by his power alone that they had hitherto continued to exist, and had been preserved in safety; and, thirdly, that it was not only once or twice that matter of joy had been afforded them, but that this existed for a long time, inasmuch as God had manifested towards them, during a long and uninterrupted period, divers proofs and tokens of his paternal favor, so that the continuance, and, so to speak, the long experience they had had of it, ought to have been the means of confirming their hope.

Footnotes

1 - Hammond reads, "We have praised God." He considers the preposition v, beth, prefixed to the name of God, as a pleonasm.

In God we boast all the day long - That is, continually or constantly. It is not a momentary or temporary expression of our feelings, but it is our habitual and constant employment. We have no other ground of reliance, and we express that reliance constantly. The word rendered "boast" here rather more literally means praise: "In God we praise all the day long." The idea is, that he was their only ground of confidence. They ascribed all their former successes to him; they had no other reliance now.
And praise thy name for ever - We do it now; we shall never cease to do it.
Selah - On the meaning of this word, see the notes at Psalm 3:2.

In God we boast - We have told the heathen how great and powerful our God is. If thou do not deliver us by thy mighty power, they will not believe our report, but consider that we are held in bondage by the superior strength of their gods.

In God we boast all the day long,.... Or, as the Targum, "in the word of the Lord", in Christ, who is God over all, and who of God is made to his church and people wisdom, righteousness, sanctification, and redemption; so that there is always matter of glorying and boasting in him;
and praise thy name for ever and ever; in this world, as long as life continues; and in the other world to all eternity; both for the works of providence and of grace; for deliverances commanded, and for salvation from all enemies wrought out.
Selah; of this word See Gill on Psalm 3:2.

thy name--as in Psalm 5:11.