Romans 4:13

¹³ For the promise that he would be the heir of the world was not to Abraham or to his seed through the law, but through the righteousness of faith.

The blessing to Abraham was not based on circumcision (verse 12) or the law, but through faith.

seed = Christ, as in Galatians 3:16 (Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ) but also to his descendants as in Galatians 3:29 (And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise).