When we speak of being one in Christ Jesus, I believe we also tap in relationship we have with Christ. that relationship determines oneness with Christ. we can’t be one with Him if we don’t live as He lived and guided us into. He lived the will of God. which lead us to the same point to live the will of God. to love one another as He loved us.
If we can make a decision to live and have LOVE and PEACE of God. we will grow up completely and become one with the head, that is being one with the messiah Christ Jesus. For the husband is the head of his wife as the messiah is the head of the church. It is He who is the saviour of the body.
He is the beginning , the firstborn of creation of God, so that He Himself might have first place in everything in the same way, even though we are many people, we are one body in the messiah, and individual parts connected to each other, Now your the messiah’s body and individual parts of it.
This tells us that in union with Him the whole building is joined together and rises into a Holy sanctuary for the Lord.
”Stop being carried away by all kinds of the unusual teachings, for it is good that the heart be strengthened by grace, not by food laws that have never helped those who follow them.”(ref, Heb 13*9