I have set watchmen upon thy walls, O Jerusalem, which shall never hold their peace day nor night: ye that make mention of the LORD, keep not silence,
I - Isaiah, speaking in the person of Messiah.
Have set watchmen upon thy walls, O Jerusalem - image from the watches set upon a city's wall to look out for the approach of a messenger with good tidings (Isaiah 52:7-8): the good tidings of the return of the Jewish exiles from Babylon prefiguring the coming return from the present dispersion (cf. Isaiah 21:6-11; Isaiah 56:10; Ezekiel 3:17; Ezekiel 33:7). The watches in the East are announced by a loud cry, to mark the vigilance of the watchmen.
Ye that make mention of the Lord, keep not silence - Hebrew, ye that are the Lord's remembrancers: God's servants who by their prayers 'put God in remembrance' of His promises (Isaiah 43:26). We are required to remind God, as if God could, which He cannot, forget His promises (Psalms 119:49; Jeremiah 14:21).