The reference is to the Holy Spirit. Not only Jesus Christ (the second person of the Trinity) comes to John, but so does the Holy Spirit (the third person of the Trinity). The Spirit overwhelmed John so that he would be able to hear and see things no human ear or eye could otherwise discern, let alone survive. See also Numbers 11:24–29; Numbers 24:2, Numbers 24:4, Numbers 24:16; 1 Samuel 10:6–10; Ezekiel 3:12, Ezekiel 3:14; Ezekiel 8:3. Without the work of the Holy Spirit we may expect John to respond to a vision from Jesus as Saul did on the road to Damascus, resulting in blindness (Acts 9:8); in that case John would not be able to record the Revelation he received.
10 I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like a trumpet