8 As We have said, it is possible for us to attain natural truth by virtue of our intellects. But all cannot do this easily; often their efforts will result in a mixture of truth and error. This is particularly the case in matters of religion and sound morals. Moreover, we cannot possibly attain those truths which exceed the capacity of nature and the grasp of reason, unless God enlightens and inspires us. This is why the word of God, "who dwells in light inaccessible,"(2) in His great love took pity on man's plight, "became flesh and dwelt among us,"(3) that He might "enlighten every man who cometh into the world"(4) and lead him not only to full and perfect truth, but to virtue and eternal happiness. All men, therefore, are bound to accept the teaching of the gospel. For if this is rejected, the very foundations of truth, goodness, and civilization are endangered.
Source: Ad Petri Cathedram (Vatican.va)