'Seed' in the Bible

Now the promises were spoken to Abraham and to his seed. He doesn't say, "To seeds," as of many, but as of one, "To your seed," which is Christ.

What then is the law? It was added because of transgressions, until the seed should come to whom the promise has been made. It was ordained through angels by the hand of a mediator.

If you are Christ's, then you are Abraham's seed and heirs according to promise.