As we have already emphasised, the rule of faith constitutes a guideline for the evaluation of the church's proclamation—which, incidentally, can already be seen in the name itself. ... In the struggle against the heresies the Fathers alluded to the regula as a summary of the authentic and only true traditio of the church. Therefore, it was the rule of faith which came first and not the heresies. One can even say that it was only through the regula that the heresies became recognizable as such. No regula no heretics. That is to say, it is the rule of faith which decides which doctrine is true and which false. The fact that in contemporary theology so little is said about “heresy” is undoubtedly connected to the fact that so little is known about a regula veritatis, by means of which talk of what is heretical becomes meaningful at all.
The designation of the regula fidei as a guideline for the evaluation of doctrine is motivated not only by the negative desire to combat false proclamation, but also by the insight that the preservation of the correct traditio is linked with struggle and decision and that the purity of doctrine is always endangered. The positive task of the regula fidei, namely, that of being the fundamentum of church doctrine, must therefore always be complemented by this other, in a certain sense “negative,” task of combating false doctrine and deciding between true and false doctrine.