Who art thou, O great mountain? before Zerubbabel thou shalt become a plain: and he shall bring forth the headstone thereof with shoutings, crying, Grace, grace unto it.
Who art thou, O great mountain? before Zerubbabel thou shalt become a plain - all mountain-like obstacles (Isaiah 40:4; Isaiah 49:11) in Zerubbabel's way shall be removed, so that the crowning topstone shall be put on, and the completion of the work be acknowledged as wholly of "grace." Antitypically, the Antichristian last foe of Israel, the obstacle preventing her establishment in Palestine, about to be crushed before Messiah, "the stone cut out of the mountain without hands," is probably meant (it is foreshadowed by Babylon, the "destroying mountain," Jeremiah 51:25; Daniel 2:34; Daniel 2:45; Matthew 21:44).
And he shall bring forth the head-stone thereof - primarily, He shall bring it forth from the place where it was chiseled, and give it to the workmen to put on the top of the building. It was customary for chief magistrates to lay the foundation and also the crowning top-stone (cf. Ezra 3:10). Antitypically, the reference is to the time when the full number of spiritual Israel, the Church, shall be completed, and also when "all (literal) Israel shall be saved" (cf. Romans 11:26; Hebrews 11:40; Hebrews 12:22-23; Rev. 7:49).
With shoutings, crying, Grace, grace unto it! The repetition expresses, Grace, perfect; grace from first to last (margin, Isaiah 26:3; Isaiah 57:19). Thus the Jews are urged to pray perseveringly and earnestly that the same grace which completed it may always preserve it. "Shoutings," of acclamation, "with a great shout ... of joy," accompanied the foundation of the literal temple (Ezra 3:11; Ezra 3:13). So shoutings, of "Hosanna" greeted the Saviour in entering Jerusalem (Matthew 21:9), when about to complete the purchase of salvation by His death: His body being the second temple, or place of God's inhabitation (John 2:20-21). So, when the full number of the saints and of Israel shall be complete, and God shall say, "It is done," then again shall "a great voice of much people in heaven" attribute all to the "grace" of God, saying, "Alleluia! Salvation, and glory, and honour, and power unto the Lord our God" (Revelation 19:1; Revelation 19:6). Psalms 118:22 regards Him as "the head-stone of the corner" - i:e., the foundation-stone. Compare the angels' acclamations at His birth, Luke 2:14. Here it is the top-stone. Messiah is not only the "Author," but also the "Finisher of our faith" (Hebrews 12:2). "Grace" is ascribed "unto it" - i:e., the stone, Messiah. Hence, the benediction begins, "The grace of the Lord Jesus Christ" (2 Corinthians 13:14).