And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
He got circumcised, showing his faith before he was circumcised. This made him a symbol for believers who aren't circumcised, so they could also receive righteousness.