Now this is something interesting I found from a website about The Trinity in the Old Testament:
“And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son. – Zechariah 12:10
‘The one they have pierced’ ties this verse in with Isaiah 53:5 and helps identify the verse as a Messianic prophecy. So God is saying that the Messiah would be Himself (Father), a spirit (Holy Spirit) and also a third male person (Jesus the Son)!
Another instance of God speaking simultaneously of Himself and someone else as one that I found new and compelling:
Then I heard the voice of the Lord saying, “Whom shall I send? And who will go for us?” – Isaiah 6:8
And the site also has this interesting observation that supports a Trinity over a Duality or Unity:
The Old Testament had a singular term for God, Eloah, which they occasionally used, but the vast majority of the time the plural form of God, Elohim, is used throughout the Old Testament.
Had there been only two persons within God, the Hebrews could have used a dual form Elohiayim, which was typically (although not always) used of things that occured in pairs.
UPDATE: Zechariah 12:10 above may be a mistranslation though.
See also these related posts: