27 What room is any longer left for doubt? The Lord Himself proclaiming that the chief commandment of the law is to confess and love the one Lord, proves Himself to be Lord not by words of His own, but by the Prophet's testimony, always signifying, however, that He is Lord, because He is the Son of God. By virtue of His birth He abides in the mystery of the one God, for the birth transmitting with it, as it did, the nature of God is not the issuing forth of another God with a different nature; and, because the generation is real, neither is the Father degraded from being Lord, nor is the Son born less than Lord. The Father retains His authority, the Son obtains His nature. God the Father is one Lord, but the Only-begotten God the Lord is not separated from the One, since He derives His nature as Lord from the one Lord. Thus by the law Christ teaches that there is one Lord; by the witness of the prophets He proves Himself Lord also.
Source: On the Trinity (New Advent)