Tertullian — on Gal 2:4 (Against Marcion Book V)

“So great had been his desire to be approved and supported by those whom you wish on all occasions to be understood as in alliance with Judaism! When indeed he says, that "neither was Titus circumcised," he for the first time shows us that circumcision was the only question connected with the maintenance of the law, which had been as yet agitated by those whom he therefore calls "false brethren unawares brought in." These persons went no further than to insist on a continuance of the law, retaining unquestionably a sincere belief in the Creator.”