Below I comment on each verse of Revelation 11:1-13. Please first read the passage in its entirety. Look up the references as we go through this.
And there was given me a reed like unto a rod: and the angel stood, saying, Rise, and measure the temple of God, and the altar, and them that worship therein.Revelation 11:1
The reed is used to make a measurement. To measure the size of something. Revelation 21:15,16. I suggest that what God is saying here is to determine the magnitude of the believers represented by the temple and the altar. These are those who we would now call Messianic believers. Compare with the following verse. Only the Jews were allowed to enter the temple. Gentiles never could.
But the court which is without the temple leave out, and measure it not; for it is given unto the Gentiles: and the holy city shall they tread under foot forty and two months.Revelation 11:2