17 Some men, indeed do not attack the truth wilfully, but work in heedless disregard of it. They act as though God had given us intellects for some purpose other than the pursuit and attainment of truth. This mistaken sort of action leads directly to that absurd proposition: one religion is just as good as another, for there is no distinction here between truth and falsehood. "This attitude," to quote Pope Leo again, "is directed to the destruction of all religions, but particularly the Catholic faith, which cannot be placed on a level with other religions without serious injustice, since it alone is true." (9) Moreover, to contend that there is nothing to choose between contradictories and among contraries can lead only to this fatal conclusion: a reluctance to accept any religion either in theory or practice.
Source: Ad Petri Cathedram (Vatican.va)