The existence of God can be proven/demonstrated from reason alone. But don’t take my word for it:
Oath Against Modernism (Denz. 2145) And first, I profess that God, the beginning and end of all things, can be certainly known and thus can also be demonstrated by the natural light of reason “by the things that are made” [cf. Romans 1:20], that is, by the visible works of creation, as the cause by the effects.