Humani Generis §4

⁴ Furthermore the human intelligence sometimes experiences difficulties in forming a judgment about the credibility of the Catholic faith, notwithstanding the many wonderful external signs God has given, which are sufficient to prove with certitude by the natural light of reason alone the divine origin of the Christian religion. For man can, whether from prejudice or passion or bad faith, refuse and resist not only the evidence of the external proofs that are available, but also the impulses of actual grace.