Dr. Couch, is Isaiah 48:16-17 about the Trinity?
Yes, indeed, it is! There is the call to listen to the voice speaking: "Come near to Me, listen to this: From the time I have not spoken in secret. From the time it took place I was there." Then the passage says: "And now the Lord God has sent Me, and His Spirit." Verse 17 adds: "Thus says the Lord, your Redeemer, the Holy One of Israel, 'I am the Lord your God, who teaches you to profit …'"
The Redeemer is the preincarnate Messiah, then there is the Spirit, and finally the Lord God who is speaking! Unger and Young point out that this is a Messianic and Trinity passage. "The type prefigures the antitype, the Servant of the Lord (42:1-4), who 'did not come alone but … his entire ministry was in the power of the Spirit, a definite Messianic concept.'" The idea is repeated in 41:14; 43:14; 49:7, 27; 54:5, 8.
Christ is called the Holy One of God who would not undergo decay, as mentioned in Psalm 16:10. Peter repeats this in Acts 3:27; 13:35.
I think we would be surprised at the number of times the Trinity is seen in both the OT and the NT.
Thanks for asking.
Dr. Mal Couch