Poole on Revelation 3:1: Remedy for Spiritual Deadness, Part 1
Verse 1: And unto the angel of the church in Sardis write; These things saith he (Rev. 1:4, 16; 4:5; 5:6) that hath the seven Spirits of God, and the seven stars; (Rev. 2:2) I know thy works, that thou hast a name that thou livest, (Eph. 2:1, 5; 1 Tim. 5:6) and art dead.
The angel of the church: see on Revelation 2:12.
[Of the Church of Sardis, ἐν Σάρδεσιν] In Sardis (Erasmus). Which belongs to the Sardinians (Beza, Piscator). Sardis was the royal seat of Cræsus according to Horace, formerly a most opulent city (Grotius).
Write: see the notes on Revelation 1:11.
[Who (it is a Periphrasis of Christ [Piscator]) has (namely, in His power [Grotius]) the seven (or, those seven [Beza, Piscator]) Spirits of God (concerning which see Revelation 1:4 [Grotius, Piscator] and 5:6, through which He attends to the salvation of men [Durham]: which He is said to have, for to Him was committed the dispensation of gifts and graces to the Church [Durham]) and the seven stars] Concerning which, Revelation 1:16, 20 (Piscator, Durham). Who holds all Bishops doing their duty most dear, and cares for them more than for others, and is able to cast away those not doing their duty.
The seven Spirits of God, and the seven stars: see the notes on Revelation 1:4, 20.
[I know thy works] Thou dost some showy things (Grotius).
I know thy works: this phrase here (as appears from what follows) can signify nothing but Christ’s comprehension of the works of this church in his understanding, not his approbation of them.
[That, etc., ὅτι, etc.] Namely, that thou art said to live (that is, thou art celebrated as if alive [Beza], thou art generally held to be a good and faithful man [Pareus, similarly Durham, Drusius, Zegers]: because they do not know thy secret things, they hold thee to be a good Christian [Grotius]: To live here is to put forth good fruits [Beza], or, to live following Christ [Grotius]), but art dead (Beza, Piscator), that is, in reality (Camerarius), spiritually, as in 1 Timothy 5:6 (Piscator); Ephesians 2:1; see on Matthew 8:22 (Grotius), that is, a hypocrite (Pareus). Thou wallowest in sin concealed from men, and such that excludes thee from my favor and eternal life. Such are called dead (Grotius). Thou dost sin by neglecting thy duty in teaching and correcting the people, averting scandal, restraining heresies (Menochius, similarly Tirinus). Thou bringest forth no fruit appropriate for one who is said to live (Drusius).
That thou hast a name that thou livest; the ministry of this church had a name, that is, were reported as famous for their faith, diligence, and holiness; but their faith, without suitable works, was dead, and they were no better than hypocrites. And art dead; spiritually dead.
 Greek: Καὶ τῷ ἀγγέλῳ τῆς ἐν Σάρδεσιν ἐκκλησίας γράψον, Τάδε λέγει ὁ ἔχων τὰ ἑπτὰ πνεύματα τοῦ Θεοῦ καὶ τοὺς ἑπτὰ ἀστέρας· Οἶδά σου τὰ ἔργα, ὅτι τὸ ὄνομα ἔχεις ὅτι ζῇς, καὶ νεκρὸς εἶ.  Cræsus, or Crœsus (595-c. 547 BC), was the king of Lydia, renowned for his wealth, and is credited with the invention of the first formal coinage system.  “To Bullatius”. Quintus Horatius Flaccus (65 BC-8 AD) was a Roman poet, perhaps the greatest of his day.  See Ephesians 4.