God, our Creator and Lord, can be known with certainty from created things by the natural light of reason.
Source (Latin):
Pius IX, Dogmatic Constitution “Dei Filius”, Session III (April 24, 1870), Canon II § 1. In: Acta Sanctae Sedis, Vol. III (1870), pp. 578–584.
Source (Systematic):
Denzinger-Schönmetzer DS 3004 / Catechism of the Catholic Church No. 47