A.T. Robertson Commentary Revelation 1:20

"the mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks are seven churches." — Revelation 1:20 (ASV)

The mystery of the seven stars (το μυστηριον των επτα αστερων). On the word μυστηριον see on Mt 13:11; 2 Thessalonians 2:7; Colossians 1:26. Here it means the inner meaning (the secret symbol) of a symbolic vision (Swete) as in 10:7; 13:18; 17:7,9; Daniel 2:47. Probably the accusative absolute (Charles), "as for the mystery" (Robertson, Grammar, pp. 490, 1130), as in Ro 8:3. This item is picked out of the previous vision (1:16) as needing explanation at once and as affording a clue to what follows (2:1,5).

Which (ους). Masculine accusative retained without attraction to case of αστερων (genitive, ων).

In my right hand (επ της δεξιας μου). Or "upon," but εν τη, etc., in verse 16.And the seven golden candlesticks (κα τας επτα λυχνιας τας χρυσας). "The seven lampstands the golden," identifying the stars of verse 16 with the lampstands of verse 12. The accusative case here is even more peculiar than the accusative absolute μυστηριον, since the genitive λυχνιων after μυστηριον is what one would expect. Charles suggests that John did not revise his work.

The angels of the seven churches (αγγελο των επτα εκκλησιων). Anarthrous in the predicate (angels of, etc.). "The seven churches" mentioned in 1:4,11. Various views of αγγελος here exist. The simplest is the etymological meaning of the word as messenger from αγγελλω (Matthew 11:10) as messengers from the seven churches to Patmos or by John from Patmos to the churches (or both). Another view is that αγγελος is the pastor of the church, the reading την γυναικα σου (thy wife) in 2:20 (if genuine) confirming this view. Some would even take it to be the bishop over the elders as επισχοπος in Ignatius, but a separate αγγελος in each church is against this idea. Some take it to be a symbol for the church itself or the spirit and genius of the church, though distinguished in this very verse from the churches themselves (the lampstands). Others take it to be the guardian angel of each church assuming angelic patrons to be taught in Mt 18:10; Acts 12:15. Each view is encompassed with difficulties, perhaps fewer belonging to the view that the "angel" is the pastor.

Are seven churches (επτα εκκλησια εισιν). These seven churches (1:4,11) are themselves lampstands (1:12) reflecting the light of Christ to the world (Matthew 5:14–16; John 8:12) in the midst of which Christ walks (1:13).