Matthew Poole and continuators on Romans 4:12

12. And the father of circumcision to them who are not of the circumcision onely, but who also walk in the steps of that faith of our father Abraham , which he had being yet uncircumcised.

The former Verse tells you he was the Father of the Believing Gentiles, for the Covenant was made with him, for all his Believing Seed, when he was Vncircumcised, which shews, That Righteousness is, and may be imputed to them also, without any outward Circumcision; and then he is the Father of the Believing Iews; especially of as many of them, as unto circumcision, do add the Imitation of his Faith. Who (besides circumcision which they deriv'd from him) do also Transcribe his divine Copy, and follow his Example of Faith and Obedience: That leave their Sins, as he did his Country; that Believe all God's Promises, and adhere to him against all Temptations to the contrary.