Albert Barnes Commentary Psalms 103:21

"Bless Jehovah, all ye his hosts, Ye ministers of his, that do his pleasure." — Psalms 103:21 (ASV)

Bless ye the Lord, all ye his hosts - His armies; the vast multitudes of holy beings, arranged and marshalled as hosts for battle, in all parts of the universe. Compare the notes at Isaiah 1:9 and Ephesians 1:21.

Ye ministers of his - The same beings referred to by the word hosts, and all others who may be employed in executing his will. The hosts or armies of the Lord are thus marshalled that they may do his pleasure, or that they may execute his purposes.

That do his pleasure - What is agreeable to him; that is, who perform his will. Employed in his service, and appointed to execute his will, they are called on to bless his name. The fact of being employed in his service is a sufficient reason for praise. It is implied here that those ministers of his actually do his will. They are obedient to his commands; they regard themselves as employed for him.