The promises were spoken to Abraham and to his descendant. Scripture doesn’t say, “descendants,” referring to many, but “your descendant,” referring to one. That descendant is Christ.
He is also the head of the church, which is his body. He is the beginning, the first to come back to life so that he would have first place in everything.
I am happy to suffer for you now. In my body I am completing whatever remains of Christ’s sufferings. I am doing this on behalf of his body, the church.
He doesn’t hold on to ⌞Christ,⌟ the head. Christ makes the whole body grow as God wants it to, through support and unity given by the joints and ligaments.