The "seven spirits [of God]" are mentioned multiple times in Revelation (KJ21):
1:4 John, To the seven churches in Asia: Grace be unto you and peace from Him who is, and who was, and who is to come, and from the seven spirits which are before His throne;
3:1 “And unto the angel of the church in Sardis write: ‘These things saith He that hath the seven Spirits of God and the seven stars: I know thy works, and that thou hast a name that thou livest, but thou art dead.
4:5 And out of the throne proceeded lightnings and thunderings and voices. And there were seven lamps of fire burning before the throne, which are the seven Spirits of God;
5:6 And I beheld, and lo, in the midst of the throne and the four living beings, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God, sent forth into all the earth.
Who or what are these seven spirits?