Jamieson, Fausset and Brown on Zechariah 14:9

9. king over all ... earth--Is 54:5 implies that this is to be the consequence of Israel being again recognized by God as His own people (Da 2:44; Re 11:15).

one Lord ... name one--Not that He is not so already, but He shall then be recognized by all unanimously as "One." Now there are "gods many and lords many." Then Jehovah alone shall be worshipped. The manifestation of the unity of the Godhead shall be simultaneous with that of the unity of the Church. Believers are one in spirit already, even as God is one (Ep 4:3-6). But externally there are sad divisions. Not until these disappear, shall God reveal fully His unity to the world (Joh 17:21, 23). Then shall there be "a pure language, that all may call upon the name of the Lord with one consent" (Zep 3:9). The Son too shall at last give up His mediatorial kingdom to the Father, when the purposes for which it was established shall have been accomplished, "that God may be all in all" (1Co 15:24).