Jamieson, Fausset & Brown

Cometh this blessedness then, &c.--that is, "Say not, All this is spoken of the circumcised, and is therefore no evidence of God's general way of justifying men; for Abraham's justification took place long before he was circumcised, and so could have no dependence upon that rite: nay, 'the sign of circumcision' was given to Abraham as 'a seal' (or token) of the (justifying) righteousness which he had before he was circumcised; in order that he might stand forth to every age as the parent believer--the model man of justification by faith--after whose type, as the first public example of it, all were to be moulded, whether Jew or Gentile, who should thereafter believe to life everlasting."