It's a name, not a title. Duke is a title, but God is a name.
There is only one God. Therefore, God is a name. A saint who has been cleansed from sin by Christ becomes deified. This means God and Man, as Jesus Christ is.
One can misappropriate God's name, becoming an idol. This is what satan did.
It is obviously both a proper noun, or name, and a functional description.
Of course this entire argument ignores theosis, which is the process of salvation in which we become sons of God by adoption, by grace what Christ is by nature, by putting on Christ in baptism and partaking of His Body and Blood so as to receive eternal life - which is something God has, and gives us, thus making us like Him, and insofar as Christ our True God put on our humanity, God became consubstantial with us, as is revealed by the names of Christ - Emanuel, meaning God with Us, and Jesus, meaning God Saves.
We must also be clear, we worship one God, but God abides in three persons: the Father, Son and Holy Spirit, God the Son, Christ our Lord, having revealed the Father to us, for we are made in the image of the Father, while God the Holy Spirit our Comforter and Paraclete indwells in us, and through Him we interact with the Trinity, for all three persons are coequal, coeternal and consubstantial, the Father unoriginate, the Son begotten before all worlds, and the Holy Spirit proceeding eternally from the Farher, and all three are fully God. Christ is also fully man, His humanity united to His deity without change, confusion, separation or division.
