“O great mountain.” In this context, the “great mountain” is not specific, but referred to any worldly or human obstacle that stood in the way of what Zerubbabel was doing in building the Temple of Yahweh. It has been suggested by some commentators that the great mountain is the mountain of rubble in Jerusalem from Nebuchadnezzar’s destruction of the city, but that does not seem likely in this context. The Judean workers were prepared to remove the physical rubble by their human strength. Also, the fact that the text reads, “Who are you, O great mountain” points to demonic and human opposition as the “who” causing the problems, and the biblical text supports that conclusion. The “mountain” in Zerubbabel’s way was mainly the people and worldly powers who tried to impede or stop the work (cp. Ezra 4:5).
When we read Zechariah 4:6-7 as a unit, we see that it is by the spirit of Yahweh that the building of the temple will be accomplished, and “not by [human] might, nor by [human] power.” Removing the physical rubble could be done with human power, but removing the human and demonic obstacles had to be done by God. In this case, the building of the Temple was not going to occur because the Judeans were such powerful people that they could force their will on their Persian overlords. The building of the Temple occurred because God was working behind the scenes to accomplish His purposes. God foretold that Zerubbabel would finish the Temple (Zech. 4:7), and he did (Ezra 6:15).
There is much that we cannot accomplish by our human power that can only be accomplished by God working with us and for us. In this case, Jerusalem and the Temple were destroyed because of human disobedience, but they could only be rebuilt by God’s power. We are wise in our endeavors if we are humble and obedient to God and always pray for His help.