To the angel[a] of the church in Sardis write:
These are the words of him who holds the seven spirits[b] of God and the seven stars. I know your deeds; you have a reputation of being alive, but you are dead.
These are the words of him who holds
These seven spirits have been and always will be before the throne. John says so in Revelation 1:4 that they are (today - present tense) before the throne. What are these seven SPIRITS?
Sealer of my redemption (Ephesians 4:30)
Confirmer of sonship/hiership (Romans 8:14-17)
Prayer warrior (Ephesians 6:17-20)
Spirit of LIFE (Romans 8:9-11)
Minister of Glory (2 Corinthians 3:4-11)
Occupant of the believer's body (temple) (1 Corinthians 3:16-17)
Discloser of truth (John 16:13-15)
John sees, in his first vision, that the seven stars are the angels of the seven churches (Revelation 1:20). This indicates that the church at Sardis is being "weighed" against the things of the SPIRIT and heavenly things of worth (angels and their worship).
I know your deeds; you have a reputation of being alive, but you are dead.
Apparently the Sardis church looks very active but much activity under a religious banner means nothing. Jesus gives us a very curious passage in Matthew 7:20-23 where people are going to claim their works (all, notice, appearing very religious or supernatural) as their tickets into heaven. Not so, Jesus says "I never knew you". All our works, including religious-appearing ones, will be tested as though by fire.... (1 Corinthians 3:12-17).
The Sardis church, whether it was the proximity of the necropolis or an internal matter, was DEAD. Sure they were active maybe even included pagan death rites in their worship, but they had lost that ALIVE, GOD-GLORIFYING, and GOD-HONORING worship.
[a] Or messenger;
[b] That is, the sevenfold Spirit