It has been pointed out that quoting Vatican I won't convince the Richard Dawkinses of the world that God can be known with certainty by the natural power of human reason.
It has also been pointed out that the Richard Dawkinses of the world don't necessarily want to be convinced.
It would be a misunderstanding of the dogma to think it means there's an argument -- call it the Babelfish Argument -- that persuades everyone who encounters it that God exists. If anything, the canon itself is directed less at atheists than at fideists who insist we can't know God exists (much less anything else about Him) except by faith. (This fideism seems to be alive and well in certain Catholic circles where it is mistaken for mature and modern faith.)
So what are we to do with the skeptic who says, "You say God can be known with certainty from the consideration of created things. Prove it!"?
For starters, I think we should try to understand what he means by, "Prove it." If he means something like, "Convince me God exists," or even, "Convince me we don't need faith to know God exists," we should point out that, going into it cold, there is no cause to be sanguine about the possibility of convincing him of anything. Again: the dogma says what is possible for reason to do, not what is necessary. If someone isn't convinced by a sound argument, that doesn't make the argument any less sound. (I'm using "argument" in a broad sense; I don't want to reduce human reason to syllogistic logic.)
But perhaps the skeptic means, "Walk me through an example of knowing God with certainty from the consideration of created things." In that case, I'd take him down the Uncaused Cause path, which in my opinion suffices to demonstrate God's existence.*
And I wouldn't stop there! As I've said before, Church teaching goes clearly beyond knowing that God exists by reason. Humani Generis opens with the observation that
absolutely speaking, human reason by its own natural force and light can arrive at a true and certain knowledge of the one personal God, Who by His providence watches over and governs the world....
Given that there is an Uncaused Cause, what can we learn about It from Its effects? I have some ideas about how to get to the "personal, "providence," and governance Pope Pius XII mentions, though I haven't given much thought to scrubbing these ideas with the wire brush of Reason Alone.
*. Yes, I know a lot of people think the Uncaused Cause is an invalid argument. See my point above about the difference between a sound argument and a convincing argument.