Ch. 1 – The church – that is, believers – makes up the body of Christ. He is the head. Let’s not forget that believers are created beings (men), made in the image and glory of God, and originally created to have dominion over all that God created. Now, with Christ as the head of that body, God’s purpose is renewed, this time with our will in line with His will through Jesus, His Son. Paul explains this in depth in ch. 2, that is, the two – the first man and the Second Man – united into one new man.
Ch. 3 – The mystery of Christ, the mystery of the Gospel, is that Gentiles/uncircumcised are fellow heirs, and are included in this one new man, an unthinkable union in Jewish minds, especially in Paul’s day.
Ch. 4 – Just as there is one new man, there are: one body, one Spirit, one hope, one Lord, one faith, one baptism, one God and Father of all.
Ch. 5 – We are to be imitators of God. As God is Three in One (Father, Son, and Spirit), so are we to be ONE, not just with God but bringing every facet of our own personhood into ONENESS, imitating God – in effect, bringing into this oneness our own individual personalities and characteristics (spiritual, emotional, physical, mental, sexual, etc.) under the headship of Jesus Christ, with the indwelling of the Holy Spirit, in the sight of God the Father.
The theme of ONENESS continues with mention of a man and a woman being one in the flesh, a very specific and clearly spelled-out Biblical doctrine of marriage between a man and a woman, which makes them them one flesh in the sight of God. Marriage, therefore, is exclusively for a man and a woman, and in the sight of God it is truly holy matrimony.