Psa 110:1 The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
Some people use this verse to support their notion of more than one person in the Godhead.
Here, LORD (the Creator) is speaking to David’s future descendant Lord (the Messiah). Even though on the surface it might seem as two “persons” engaged in a conversation, when taken in context it can be understood that it is a prophetic declaration.
The Bible clearly says that Sonship had a clear beginning in time. God did not always exist in the flesh.
Gal 4:4 But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
This verse declares that Sonship came into existence only “when the fulness of the time was come”. Also, the Son was “made under the law” meaning that the flesh came into existence only after the law was given by Moses.
Again, the Bible points out,
Heb 1:5 For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?
The statement made here is “I will be to him a Father” invoking the future tense. Not “I am already his Father”. Also, the verse declares “He shall be to me a Son”.
Coming back to Psa 110:1, Jesus did not exist in the flesh when it was written. But God had already planned mankind’s salvation even before he created them. That’s why it is written that Jesus is the “Lamb slain from the foundation of the world”.
Isa 46:9 “… I am God, and there is none else; I am God, and there is none like me, 10 Declaring the end from the beginning, and from ancient times the things that are not yet done, saying, My counsel shall stand, and I will do all my pleasure:”
So, the “LORD speaking to the Lord” is a literary prophetic tool. The Bible does very well to lay to rest any erroneous interpretations by declaring in the final chapter of the Bible:
Rev 22:16 I am the root and the offspring of David, and the bright and morning star.
As the LORD, He created Adam and became the “root” or the “source” of David.
As the Lord, He was born in David’s blood line and became the “offspring” of David.
While the Bible does distinguish between the distinct “offices” of the Father, Son and the Holy Ghost, nowhere does it say that there are three “persons” who unite to become one God. Rather, the Only God who created the heavens and the earth came in the flesh himself to save his people (Matt 1:21 thou shalt call his name JESUS: for he shall save his people from their sins.”) He resurrected from the dead and now lives in us as the Holy Ghost.