If, then, these things are so, it is in the same degree manifest that all doctrine which agrees with the apostolic churches—those moulds and original sources of the faith must be reckoned for truth, as undoubtedly containing that which the (said) churches received from the apostles, the apostles from Christ, Christ from God. Whereas all doctrine must be prejudged as false which savors of contrariety to the truth of the churches and apostles of Christ and God. It remains, then, that we demonstrate whether this doctrine of ours, of which we have now given the rule, has its origin in the tradition of the apostles, and whether all other doctrines do not ipso facto proceed from falsehood. We hold communion with the apostolic churches because our doctrine is in no respect different from theirs. This is our witness of truth.
Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XXI