John 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.I will argue that that author of the passages in the Old Testament that testify of Christ, knew Christ or at least knew about him. There are many examples like the life of Joseph or the second Psalm or Isaiah 53 that are obvious, but as you see Christ more and more in scripture you ask, is Christ the main character even of the Old Testament?
What books do we consider to be scripture?
The books that the councils of the 2nd and 3rd centuries held to be scripture. Note that the Ethiopian church has the most number of books in scripture - 84.
The author of the Pauline epistles is, well, Paul who was inspired by God. Paul wrote as a human inspired by God.
The author of the passages in the Old Testament that testify of Christ - whether Moses or Ezekiel or Isaiah or Daniel didn’t KNOW Christ as we know Him, they knew Him as the Word of God, as God in short.