10 I was in spirit in the Lord’s day, and I heard behind me a loud voice, as of a trumpet
in spirit – the phrase “spirit” without the Greek article after the Day of Pentecost describes the anointing of the spirit of God. The expression “in spirit” is also used in Rev 4:2, “I became as in spirit, and behold a throne was standing in heaven.” This is similar to Ezek 3:12,14; 8:3; 11:1,24; 43:5 which describes the anointing of the Spirit of Christ that lifted up Ezekiel. The Spirit of Christ was the Holy Spirit of the Old Testament.
in the LORD’s Day – in the Greek language, one is “in” a time, and not “on” a time, as in English. So John became as “in spirit in the LORD’s Day.” In Rev 4:2, he became as “in spirit” before a throne, meaning that he was “transported” by the spirit of God, into that time.
a loud voice as a trumpet – the symbolism of the trumpet, tells of the LORD’s Day, see Mat 24:31. In the New Testament, the trumpet is almost exclusively used to signify the last days, and the last trumpet, signifies the LORD’s Day. Paul told us that the Lord “will descend with the voice of the Archangel and the trumpet of God” (1Thes 4:16). Jesus said that the dead will “hear his voice.” (John 5:28)