Session III, c. 2 of Vatican I argues that by reason, God can be clearly known, but also that revelation was given so that God could be known by everyone with firm certitude (read the whole thing, I’m summarizing here). The students pounced on this as a contradiction: how can one add firm certitude to something already clearly known?
There are two options:
1.) Firm certitude adds nothing to what is clearly known. i.e. the two are synonymous and interchangeable. In this case we would have to read the certitude clause entirely in conjunction with the previous clause. Before revelation, some men could have certitude/ clear knowledge about God, and afterwards everybody could. Or perhaps the sense is this, though by reason God could be clearly known by some, revelation does not make God known to everybody by watering down the certitude that could be achieved. Revelation does not, say, dumb down natural reason into a popular doctrine that is easier to understand but less intrinsically compelling.
2.) Firm certitude adds something to what is clearly known. Perhaps the clarity of reason, at least about God, never rises to the level of a firm certitude. We see things clearly, but in a light that we know is only human; or perhaps we realize that the clarity we can reach by natural reason is a delicate thing, not an unshakable state that could stand up to any and every assault. “Firm” (like “solid”) is a state that is relative to the forces that are acting on a thing: the same rope-bridge that a walking man would consider very firm might be considered flimsy by a man who want to ride an elephant across it. Perhaps clearly is said relative to our own power of vision whereas with firm certitude is said relative to the calamities or assaults that might befall our knowledge.