"The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool." (Psalm 110:1)
The 110th Psalm is one of the most significant of the so-called Messianic Psalms, prophesying of Christ a thousand years before He came. Its very first verse should completely settle the question as to whether or not the Old Testament teaches that there is only one person in the Godhead, since it recounts an actual conversation between at least two Persons of the Godhead. This first verse is quoted, in whole or in part, at least five times in the New Testament and was even used by Christ Himself (Matthew 22:41-46) to prove His own deity.
Two of the Hebrew names for God are used in this verse: "Jehovah said unto Adonai. . . ." The name Jehovah is used again in Psalm 110:2-4, and Adonai in verse 5. God, in the person of Adonai, has gone to earth on a divine mission to save His people, but has been repudiated by His enemies on earth. Accordingly, God, in the person of Jehovah, invites Him back to heaven for a time, where He will be at His right hand until it is time for Him to return to earth to rule, striking through all opposing "kings in the day of his wrath" (v. 5).
In this coming "day of thy power" (v. 3), "thy people shall be willing." The word here is actually the word for "free will offerings." They will be as priests offering their own lives to Him as freewill offerings when they finally recognize Him as their Messiah/King and eternal High Priest (v. 4).
Now, although this prophecy applies specifically to the Second Coming and the future conversion of Israel, there is a beautiful secondary application used in Scripture for His people right now. "I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service" (Romans 12:1). "Seek those things which are above, where Christ sitteth on the right hand of God" (Colossians 3:1). HMM