Matthew Henry Presbyterian @wholebiblehenry

We have now come to that glorious vision which the apostle had of the Lord Jesus Christ, when he came to deliver this revelation to him, where observe, I. The account given of the person who was favoured with this vision. He describes himself, 1. By his present state and condition. He was the brother and companion of these churches in tribulation, and in the kingdom and patience of Christ.

Commenting on Revelation 1:9-20

Geneva Bible Notes Reformed @genevanotes

I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet, (h) This is a holy trance expressed, with which the prophets were entranced, and being carried out of the world, conversed with God: and so Ezekiel says often, that he was carried from place to place by the Spirit, and that the Spirit of the Lord came on him.

Jamieson-Fausset-Brown Reformed @jfbcommentary

I was--Greek, "I came to be"; "I became." in the Spirit--in a state of ecstasy; the outer world being shut out, and the inner and higher life or spirit being taken full possession of by God's Spirit, so that an immediate connection with the invisible world is established. While the prophet "speaks" in the Spirit, the apocalyptic seer is in the Spirit in his whole person.