Oecumenius

When John said, "I was in the Spirit" indicates a vision not perceptible to the senses, nor seen by the flesh or eyes, but rather by prophetic insight; concerning these spiritual perceptions Isaiah said: "He gave me understanding early on, He gave me an ear to hear, and the instruction of the Lord opened my ears." (Isa. 54:4)

Commentary on Revelation ↗

Dionysius bar Salibi

I was in the Spirit, on the day of the Lord—He calls the first day of the week the Lord's, because on that day our Lord rose from the tomb, and on that day the Spirit descended upon the apostles, and on that day creation was created, and on that day our Lord arose.

Commentary On The Apocalypse 1:10 ↗

Bede

I was in the Spirit on the Lord's day. He also indicates a suitable time for the spiritual vision. For Scripture often expresses the fixed limits of causes, as often of place, body, or air, so also of time. For the angels visit Abraham at noon, Sodom in the evening. Adam feared at the voice of the Lord walking in the afternoon. And Solomon received wisdom at night, which he would not keep.

Commentary on Revelation ↗

Alcuin of York

I was in the spirit on the Lord's day. Here we need to start speaking about species and genus individually. John's spirit did not leave his flesh altogether when he saw this, because his body could not have survived without spirit; but, being in ecstasy, he could not feel or understand anything through his body. …

Commentary On Revelation ↗