Albert Barnes on Revelation 3:1

And unto the angel of the church in Sardis. Rev 1:20.

These things saith he that hath the seven Spirits of God. Rev 1:4. If the phrase, "the seven spirits of God," as there supposed, refers to the Holy Spirit, there is great propriety in saying of the Saviour, that he has that Spirit, inasmuch as the Holy Spirit is represented as sent forth by him into the world, Jn 15:26-27, 16:7,13-14. It was one of the highest characteristics that could be given of the Saviour to say, that the Holy Ghost was his to send forth into the world, and that that great Agent, on whose gracious influences all were dependent for the possession of true religion, could be given or withheld by him at his pleasure.

And the seven stars. Rev 1:16. These represented the angels of the seven churches, (Rev 1:20) and the idea which the Saviour would seem to intend to convey here is, that he had entire control over the ministers of the churches, and could keep or remove them at pleasure.

I know thy works. Rev 2:2.

That thou hast a name that thou livest. Thou dost profess attachment to me and my cause. The word life is a word that is commonly employed, in the New Testament, to denote religion, in contradistinction from the natural state of man, which is described as death in sin. By the profession of religion, they expressed the purpose to live unto God, and for another world; they professed to have true, spiritual life.

And art dead. That is, spiritually. This is equivalent to saying that their profession was merely in name; and yet this must be understood comparatively, for there were some even in Sardis who truly lived unto God, Rev 3:4. The meaning is, that, in general, the profession of religion among them was a mere name. The Saviour does not, as in the case of the churches of Ephesus and Thyatira, specify any prevailing form of error or false doctrine; but it would seem that here it was a simple want of religion.

(a) "seven spirits" Rev 5:6 (b) "know" Rev 2:2 (C) "livest" 1Timm 5:6