Zacarias 4:7

O great mountain? — This is figurative of the colossal difficulties put in the way of the completion of the building of the Temple by the neighbouring powers. (Comp. Mateus 21:21.)

Thou shalt become a plain. — This certainly gives the true meaning of the original, which, however, is singularly graphic, and consists of but one word — literally, to a plain: i.e., thou shalt become. LXX. mistake the word for an Aramaic infinitive, and render τοῦ κατορθῶσαι, “that thou shouldest bring it to a successful issue.” In the preceding vision, Joshua, as the high priest — in this, Zerubbabel, as the Prince of Judah — is the representative of the nation; in Zacarias 4:14 the two are referred to simultaneously.

Grace, grace unto it — i.e., unto the head stone which, as being the crowning stone of the building, is used to represent the whole Temple. The words are a prayer, which takes the form of a shout of triumph (like Hosanna!), and mean, May God’s grace or favour rest on the house for ever!