Albert Barnes Commentary Psalms 145:21

"My mouth shall speak the praise of Jehovah; And let all flesh bless his holy name for ever and ever." — Psalms 145:21 (ASV)

My mouth shall speak the praise of the Lord—That is, I will utter his praise. In view of all these things, in view of his character and works, I will praise him. I will be found among those who honor him; who acknowledge him as the true God; who render homage for what he is, and thanks for what he has done.

And let all flesh bless his holy name forever and ever—All people; all that dwell upon the earth. Let his praise be universal and eternal. This is the language of true piety; all in whose heart there is any true religion will heartily say Amen. No desire of a pious heart is more constant and strong than that God should be praised, adored, honored by all intelligent creatures; that he should be known and acknowledged in all the earth as the true God; that his praise should ascend from all parts of the universe forever. See the notes at Psalms 100:1-5.