Vatican I declared infallibly that God can be clearly known by rational proof but did not give the proof or even confirm that we had found one yet. This is, in fact, all they could have done – they can tell philosophy that it’s taken a wrong turn and committed a theological mistake but they can’t tell it how to do its job.
These sorts of decrees are usually taken as undermining the credibility of Christian philosophers by determining the results of their inquiry in advance. This is true up to a point, but it misses the more important point that by not advocating any particular theistic argument the Church is allowing for the possibility that all the arguments we presently have are fallacious and so are impediments that need to be broken down and thrown out of the way.
In other words, for all we know Vatican I gave a dimension of religious fervor to the criticism of natural theology. Philosophers can’t borrow any light from its infallible decrees to give them a clue about what to think of the cosmological argument or the role of religious experience or the transcendental capability of the principle of causality or Berkeleyan All-is-perceived-in God arguments, etc. For all one can tell from reading Vatican I, natural theologians are in the state of third-century astronomers arguing over which circular orbits best describe the motions of the sun and planets.