Albert Barnes Commentary Zechariah 14:9

"And Jehovah shall be King over all the earth: in that day shall Jehovah be one, and his name one." — Zechariah 14:9 (ASV)

And the Lord shall be king over all the earth - Such should be the influence of the living water, that is, of the Spirit of God. God who has ever reigned and will reign, a great King over all the earth (Psalms 47:3, Psalms 47:8), shall be acknowledged by His creatures as what He is.

There shall be one Lord - More exactly, The Lord shall be One, and His Name One. He had prophesied before, I will cut off the names of the idols out of the land (Zechariah 13:2). The Church, being thus cleansed, no other lord or object of worship will be named but Himself. This is one of those prophecies of continued expansion and development, ever bursting out and enlarging, yet never, until the end, reaching its full fulfillment.

Dionysius states: “Since in this life we contemplate God in His effects, in which His whole perfection does not shine forth, we now know Him obscurely and imperfectly, His perfections being represented in various ways by various things. In our home we shall see Him as He is, face to Face, through His Essence. Therefore, He will then be represented by one name, as He shall be beheld by one gaze.”