For shall be, read are. The strength of the fortress of Jerusalem should be the saving of Judah, but that strength would depend on the protection of “the Lord of Hosts, their God.”
Related Commentaries of Zechariah 12:5
Zechariah 12:5
5 And the governors of Judah shall say in their heart, The inhabitantsb of Jerusalem shall be my strength in the LORD of hosts their God.