Just as God is three and one as God, so we are as human beings are one with God in Jesus Christ. We are not of course ‘trinities’ but we participate in the oneness of God in the pattern that is known as perichoresis. In this state of being we are one with God and in being so, we are fully ourselves. This is the meaning of sonship and what it is to be ‘in the one spirit.’
‘But whoever is united with the Lord is one with him in spirit’ 1 Cor 6.17 NIV.
The oneness that is the trinitarian God is the oneness that we have been drawn into as sons and daughters of God. The two have been made one is not just about Jews and Gentiles. It’s about our achieved union with God.
Thomas Torrance speaking of the church fathers and the Nicaean Creed, observes, “The homoousios* was harnessed to the Gospel of salvation proclaimed in the New Testament and linked it to their belief ‘in one God the Father Almighty, Maker of heaven and earth and of all things visible and invisible.’ It was through this two-way reference that it acquired increasing spiritual authority for it supplied the Church with a firm conceptual grasp of the central truth of the incarnate economy of redemption which Christ undertook for our sake.” (1)
The parameters of The Creed are normative for the Christian church. It defines what it means to be Christian and the kind of gospel that is a true gospel and not a made-up aberration that reinstates the law while subverting the incarnation and extinguishing the Kingdom.
*Homoousion is a Christian theological term, most notably used in the Nicene Creed for describing Jesus as "same in being" or "same in essence" with God the Father. The same term was later also applied to the Holy Spirit in order to designate him as being "same in essence" with the Father and the Son.Wikipedia (1) Torrance, Thomas F.. The Christian Doctrine of God, One Being Three Persons (T&T Clark Cornerstones) (p. 94). Bloomsbury Publishing. Kindle Edition.