In my reading of Isaiah I just came across a passage that is as explicitly Trinitarian as any that you will find in the New Testament (my friend Glen Scrivener should really appreciate this, not that he’s unaware of this passage):

“I, even I, have spoken and called him; I have brought him, and he will prosper in his way. 16. Draw near to me, hear this; from the beginning I have not spoken in secret, from the time it came to be I have been there. And now the Lord GOD has sent me, and his Spirit. 17. Thus says the LORD, your Redeemer, the Holy One of Israel: I am the LORD your God, who teaches you to profit, who leads you in the way you should go.” ~Isaiah 48:15-17 (ESV)