This is a gross abuse of language. The "infallible teacher" is the means by which dogma is produced. Dogma is the ends. It is an abuse of language to refer to the means as the "form" and the ends as the "matter".
The infallible teacher not only "makes" the dogma - i.e., formulates the dogmatic proposition;
it also infallibly proposes it to us for belief. Those are two distinct acts.
So, the infallible teacher is not only the
means by which the dogma is "produced"; it is also the infallible teacher that proposes it to us for belief. In the latter sense, the infallible teaching authority is the proximate
motive for belief; the dogmatic proposition that it "made" is the matter
to be believed.
The matter (dogma) is
what we believe; the infallible teacher is
why we believe it.
But as said above in a previous reply to Ladislaus, the truth that dogma constitutes the proximate rule of faith is evident in the definition of heresy. The best of all definitions, because it is the most intelligible, is the essential which provides the proximate genus and the essential difference. A heretic is a baptized Catholic who denies dogma.
But how do we
know what teachings are dogmas? That is, how do we know what propositions are revealed truths that requires the assent of Divine and Catholic Faith? Answer: We know what propositions are dogmas, because the infallibly teacher told us. Without the infallible teacher, every person would be left on his own to determine what truths have been revealed by God.
I agree with you that dogmas are the matter of the proximate rule of faith (
what is believed); but there is also no doubt that the infallible teacher is the formal aspect of the proximate rule of faith, since the infallible teaching authority is the proximate
motive for belief.