In Russia there is an old and popular toy, the matryoshka or nesting dolls. When taken apart, they reveal surprise layers of delightful, and ever smaller, hand painted dolls. This is an excellent illustration of the layers of understanding in solving a mystery.
The concept of one flesh is just such a mystery, It is hidden within a mystery that has been under explored and under appreciated as a revelation about the nature of relationship between humanity and God, Christ and His Church, and between husbands and wives.
In the beginning, God was in essence ONE FLESH, the God of Creator-Redeemer-Comforter called variously the Trinity or the Godhead or the PARENT-CHILD, or simply GOD.
This ONE FLESH GOD said "let us make humans in our image: male and female...."
Therefore, the Bible says "for this reason."...a man shall leave his family and cling to his woman...a woman will move from the allegiance of her birth family to this new unit...they shall be "ONE FLESH".
The one flesh was the standard concept for explaining a uniting in which two elements combined. It was the reuniting of separates that were unique in, and of themselves, but whose completeness was best realized in a reunion of the halves to make a new whole.
In the New Testament, Jesus reaffirms the importance of this marital relationship by repeating that the people involved shall be "ONE FLESH".
Because that ideal of unity, that concept of the two halves becoming once more a whole, was a fundamental concept to His own teaching about relations with "GOD."
It was the very relationship, in fact, that he wanted to have with his "BRIDE" (the term referring to those followers who would become the Church) and what he conveyed repeatedly.
Paul indicates this with his own repeated imagery of the "Body” as diverse parts all working in harmony and coordination. He indicates it with his discussion of husbands and wives. He indicates this with his imagery of the 'ev Christos' of Galatians. There in Galatians he paints the picture of moving from one sphere of existence into a new, totally different sphere, where the believer merges, integrates, blends, disappears into, becoming the new creation of Jesus Christ.
All imagery of the same ONE FLESH revealed first in creation repeatedly since the creation account.