A.T. Robertson Commentary Revelation 1:10

"I was in the Spirit on the Lord`s day, and I heard behind me a great voice, as of a trumpet" — Revelation 1:10 (ASV)

I was in the Spirit (εγενομην εν πνευματ). Rather, "I came to be in the Spirit," came into an ecstatic condition as in Ac 10:10f.; 22:17, not the normal spiritual condition (εινα εν πνευματ, Romans 8:9).

On the Lord's Day (εν τη κυριακη ημερα). Deissmann has proven (Bible Studies, p. 217f.; Light, etc., p. 357ff.) from inscriptions and papyri that the word κυριακος was in common use for the sense "imperial" as imperial finance and imperial treasury and from papyri and ostraca that ημερα Σεβαστη (Augustus Day) was the first day of each month, Emperor's Day on which money payments were made (cf. 1 Corinthians 16:1f.). It was easy, therefore, for the Christians to take this term, already in use, and apply it to the first day of the week in honour of the Lord Jesus Christ's resurrection on that day (Didache 14, Ignatius Magn. 9). In the N.T. the word occurs only here and 1 Corinthians 11:20 (κυριακον δειπνον θε Λορδ'ς Συππερ). It has no reference to ημερα κυριου (the day of judgment, 2 Peter 3:10).

Behind me (οπισω μου). "The unexpected, overpowering entrance of the divine voice" (Vincent). Cf. Ezekiel 3:12.

Voice (φωνην). Of Christ, as is plain in verses 12f.

As of a trumpet (ως σαλπιγγος). So in 4:1 referring to this.

Saying (λεγουσης). Present active participle genitive case agreeing with σαλπιγγος rather than λεγουσαν, accusative agreeing with φωνην. So on purpose, as is clear from 4:1, where λαλουσης also agrees with σαλπιγγος.