Romans 4

“And he received the sign of circumcision, a seal of the righteousness of the faith which he had while uncircumcised, that he might be the father of all who believed … (v11).  Abraham was counted righteous by God because of his faith (v2).  His obedience (in the form of circumcision) came afterward, as a “seal” or outward mark which reflected his trust in God.  Therefore, he is the father not just of those who keep the Law, but “of all who believe.”

We all come to God by faith through grace.  Our obedience afterward, as His children, is a sign of our love for Him (John 14:15).