The unity of God is the foundation truth of the Old Testament, as it was the central testimony for which the Jewish people were responsible in a world everywhere else given over to idolatry.
But during the Jewish economy, God, though known to be One, was not known as He is.
It was only Jesus who made Him truly known, as we see by that act of incomparable grace in which He was fulfilling all righteousness when baptized of John in Jordan. There, as the Holy Spirit descended on Him, the Father from heaven proclaimed Him to be His beloved Son. The Trinity stood revealed. It is in the Persons of Father, Son and Holy Spirit that God, the one God, is really known. Without Jesus this was impossible; when He takes the first step, the Trinity in unity shines out-love and light, wherein is no darkness at all. How infinite is our debt to the Word made flesh, who deigned to tabernacle with us, only-begotten Son who declared God and revealed the Father! [16]