Revelation 1:10 I was in the Spirit on the Lord’s Day, and I heard behind me a loud voice, as of a trumpet,

What does it mean to you to be “in the spirit”?
Is it a trance?
Is it a vision?
Is it an overwhelming presence of God?
I think it could be any of these things or all of them.
Certainly, John will see visions as given by the Lord.
He marks the day as “the Lord’s Day”.
This is not the Sabbath or John, being Jewish, would call it the Sabbath.
The disciples of the Apostles and later church fathers often wrote about the Lord’s Day and the meaning leans towards the first day of the week, the day the Lord resurrected from the dead.
This helps give a greater meaning to the title Alpha and Omega, the First and Last, used again in the next verse.
Jesus is the Sunday through the Saturday, the First and Last day of the week.
He is all time, space, and matter.
As Paul said, “He is the image of the invisible God, the firstborn over all creation. For by Him all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him. And He is before all things, and in Him all things consist.” Col 1:15-17
Finally, John says he heard a loud voice, as of a trumpet, behind him.
This is where you will find the Lord – behind you.
Isaiah says the God of Israel will be your rear guard (Isa 52:12) and the glory of the Lord shall be your rear guard. Isa 58:8
His voice sounds as of a trumpet.
Throughout Jewish history, the shofar (trumpet) is used to sound a special day or assembly.
It is a sound that demands your attention to the service and worship of God.