Galatians 2:8

He that wrought for Peter unto the apostleship of the circumcision (o gar energhsa Petrwi ei apostolhn th peritomh). Paul here definitely recognizes Peter's leadership (apostleship, apostolhn, late word, already in Acts 1:25 ; 1 Corinthians 9:2 ) to the Jews and asserts that Peter acknowledges his apostleship to the Gentiles. This is a complete answer to the Judaizers who denied the genuineness of Paul's apostleship because he was not one of the twelve.