I remember a talk I saw by Scott Hahn on EWTN a long while back where he explained this in an interesting way. Let's say that using only my reason or intellect, I concluded that all the doctrines of Catholicism were true. That would amount to nothing more than a nice coincidence that God and I happened to agree. On the other hand, you must have faith whereby we believe these things because God has revealed them, not solely because you personally judged them to be true. In addition to that, faith must be taken to heart--we need that true metanoia (a change of heart from sin to the practice of virtue)--which the Pope seems to be referring to here.
Of course, in relality, due to our fallen nature, using just our intellect, we tend not to even have that nice coincidence where we agree with God. Without that metanoia, we tend to oppose God. Therefore, as the Pope notes, we end up imposing on our religion our erroneous ideologies which we have derived ourselves or which we have taken from other sources other than God, but which please our fallen intellects. We become "ever learning yet never attaining knowledge of the truth." (2 Tim 3:7).
It should also be noted, the opposite thing, eliminating the intellect in the realm of religion, causes the same problems. Reducing faith to merely a kind of motion of the heart, hidden and unconscious, a natural instinct belonging to the emotions, a feeling for the divine that cannot be expressed in words or doctrinal propositions is the heresy of Modernism. By denying the reality of an objective, external revelation received with the intellect ("faith comes by hearing" Rom. 10:17), the door is opened again for the imposition of all sorts of false ideologies incompatible with that revelation.
