John Calvin Commentary Psalms 103:21

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

Bless Jehovah, all you His hosts. By hosts should not be understood the stars, as some explain it. The subject of the preceding verse continues. Nor is the repetition superfluous, for the word hosts teaches us that there are myriads of myriads who stand before the throne of God, ready to receive every indication of His will.

Again, they are called His ministers who do His pleasure, to indicate to us that they are not there intent on idly beholding God’s glory, but that having been appointed as our ministers and guardians, they are always ready for their work. Instead of word, the term pleasure is here used, and both are employed with much appropriateness; for although the sun, the moon, and the stars observe the laws which God has ordained for them, yet being without understanding, they cannot properly be said to obey His word and His voice.

The term obey is indeed sometimes transferred to the mute and non-sentient parts of creation. It is, however, only in a metaphorical sense that they can be said to listen to God’s voice, when by a secret instinct of nature they fulfill His purposes. But this in the proper sense is true of angels, who actively obey Him once they understand from His sacred mouth what He would have them do.

The word pleasure expresses more clearly a joyful and cheerful obedience, implying that the angels not only obey God’s commandments but also willingly and with the greatest delight receive the indications of His will, that they may perform what He would have them do. Such is the meaning of the Hebrew noun, as has been stated elsewhere.