Albert Barnes Commentary Zechariah 8:20

"Thus saith Jehovah of hosts: [It shall] yet [come to pass], that there shall come peoples, and the inhabitants of many cities;" — Zechariah 8:20 (ASV)

It shall yet be that – The promises are those which God had already made by Isaiah (Isaiah 2:2 and following) and Micah (Micah 4:1 and following). Yet where was any sign of their fulfillment? The Jews themselves were but a handful: the temple remained unfinished, its completion depending, from a human perspective, on the will of their pagan masters; the rival worship in Samaria was established, inviting coalition. Appearances and experience were against it.

God essentially says that it was, from a human perspective, contrary to all expectations. But “weakness is always Heaven’s might.” In spite of all—the small number of those who had returned, their downheartedness, broken condition, and hopelessness—though all had failed until then, though (or rather because) all human energy and strength were gone, as God had said before, The Lord shall yet choose Jerusalem (Zechariah 1:17;Zechariah 2:16 (2:12 in English versions)), so now, it shall “yet” be “that.”

Nations and many cities shall come – He vividly describes the eagerness and mutual impulse with which not only many but mighty nations would throng to the Gospel, and how every new conversion would win others as well, until the great tide would sweep through the world.