“The harmony of faith and reason” or just “faith and reason” has the minimal sense that the faith is not absurd, or perhaps that some sort of case can be made for it. But a much more interesting sense consists in claiming that the silence of revelation is not exclusion. In fact, the content of the revelation features the results of reasoning as an essential and intrinsic component. So what if Jesus never called the Father ipsum esse subsistens? That’s still who he is. So what if the author(s) of Genesis couldn’t have thought of creation ex nihilo? That’s what happened. So what if Trent didn’t describe transubstantiation through phenomenology or existentialist categories? That might be the insight by which we finally see what it (Trent? transubstantiation?) means.
Absent this, I don’t see much that can remain of orthodox Christianity. Even basic accounts of Christ or God demand philosophical concepts (nature/person) that were unknown until centuries after there were Christians, Apostles, or even Christ himself.
Seen from this angle, the Catholic Church is making a more radical claim than just one to be an authoritative interpreter of doctrine, they are setting themselves up as gatekeepers of what purely rational and human insights are intrinsic to supernatural revelation. The analogy to the Incarnation is unmistakable, which seems to serve as the ultimate foundation for the harmony of faith and reason.