Galatians 3:16

His seed; Christ, as the head of his church; and through him, all believers, who constitute his body.

He saith not, And to seeds, as of many; he does not make the promise to Abraham's seeds, as if he were speaking of the many individual children of Abraham; in other words, were making the promise to each one of the many who are his children by outward descent.

But as of one, And to thy seed; he makes the promise, as speaking of one, to one seed of Abraham.

Which is Christ; that is, this one seed that receives the promise is Christ, and in him all believers, who constitute his body.