Not every Christian is a born again believer
Jesus likened himself to the true vine, and every man and woman who profess to believe in him as individual branches in the vine.
John 15:1-2 "I am the true vine, and my Father is the husbandman. 2 Every branch in me that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit."
Grafting is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. One of the most important factors for a successful graft is that the scion and the stock must be genetically compatible. As a general rule, the closer two plants are genetically, the more likely the graft union will form. Almonds and apricots graft well onto peach rootstocks, but an apricot scion is not compatible with almond stock nor is an almond scion with apricot stock.
The genes that is in the branches must be compatible with the genes in the true vine (Jesus Christ) so that the branches can continue to live off the life that is in the vine. The branches in the vine may not be the same individually, just like the tree of life which bare 12 manner of fruits.
Revelation 22:2 "In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations."
God has gifted each and every believer differently, but they are born again by the same Spirit of God. Brethren in the body of Christ means "brothers and sisters" related by the "seed" which is the word of God (Luke 8:11). "Seed" is a DNA term, if I may put it this way. It contains the specific genetic information of how an embryo would develop and grow into adulthood.
In the case of a born again Bible-believing Christian, the "seed" contains the genetic information of how our "new man" would develop into the image of Jesus Christ ("the Word of God"). Therefore, every member of the body of Christ is given the title "son of God", even as Jesus Christ is the "Son of God". When we address someone as "brethren", that means that he or she is of a common faith along with us.
There is a common thing in every brethren, that is, "the word made flesh" in the earthen vessel of every born again believer. This new man is birthed by the same word of God sown in an honest and good heart of different individuals. The "body of Christ" is the gathering together of every "new man" into one body, and it is from the church that the preaching of the gospel goes forth into the whole world.
While Jesus Christ is the true vine, not everyone who profess to believe in him is genetically compatible with the life that is in the vine. Not everyone who profess to believe in Jesus Christ believe in the word of God. Not every Christian is a born again believer.