All Doctrine True Which Comes Through the Church from the Apostles, Who Were Taught by God Through Christ. All Opinion Which Has No Such Divine Origin and Apostolic Tradition to Show, is Ipso Facto False
From this, therefore, do we draw up our rule. Since the Lord Jesus Christ sent the apostles to preach, (our rule is) that no others ought to be received as preachers than those whom Christ appointed; for “no man knows the Father save the Son, and he to whomsoever the Son will reveal Him.” Nor does the Son seem to have revealed Him to any other than the apostles, whom He sent forth to preach— that, of course, which He revealed to them. Now, what that was which they preached— in other words, what it was which Christ revealed to them— can, as I must here likewise prescribe, properly be proved in no other way than by those very churches which the apostles founded in person, by declaring the gospel to them directly themselves, both vivâ voce, as the phrase is, and subsequently by their epistles.
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 savours 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.
Source: The Prescription Against Heretics (New Advent)