The promises were made to Abraham and to his offspring. He did not say, "and to descendents," as if to many, but instead, as if to one, he said, "and to your offspring," who is Christ.
For now I rejoice in my passion on your behalf, and I complete in my flesh the things that are lacking in the Passion of Christ, for the sake of his body, which is the Church.
and not holding up the head, with which the whole body, by its underlying joints and ligaments, is joined together and grows with an increase that is of God.