and the forefather of the circumcised, namely of those who not merely are circumcised, but also walk in the steps of the faith which our forefather Abraham had while he was as yet uncircumcised.
In other words, it is not the children by natural descent who count as God's children, but the children made such by the promise are regarded as Abraham's posterity.
Through faith Noah, being divinely taught about things as yet unseen, reverently gave heed and built an ark for the safety of his family, and by this act he condemned the world, and became an heir of the righteousness which depends on faith.
But, in this instance, one who does not trace his origin from them takes tithes from Abraham, and pronounces a blessing on him to whom the promises belong.