When men, therefore, are not Christians even on their own admission, how much more (do they fail to appear such) to us! What sort of truth is that which they patronize, when they commend it to us with a lie? Well, but they actually treat of the Scriptures and recommend (their opinions) out of the Scriptures! To be sure they do. From what other source could they derive arguments concerning the things of the faith, except from the records of the faith?
Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XIV