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.
Canon 2.1 from Vatican I, Session III (Denz. 1806)
If anyone shall have said that the one true God, our Creator and our Lord, cannot be known with certitude by those things which have been made, by the natural light of human reason: let him be anathema.
Vatican I, Session III (Denz. 1785)
The same Holy Mother Church holds and teaches that God, the beginning and end of all things, can be known with certitude by the natural light of human reason from created things; “for the invisible things of him from the creation of the world, are clearly seen, being understood by the things that are made [Romans 1:20]
Decree of the Sacred Congregation of the Index, II, 15 June, 1855 (Denz. 1650)
Reason can prove with certitude the existence of God, the spirituality of the soul, the freedom of man.
Theses written by Louis Eugene Bautain under order of his bishop, Sept. 8, 1840 (Denz. 1622)
Reason can prove with certitude the existence of God and the infinity of His perfections. Faith, a heavenly gift, is posterior to revelation; hence it cannot be brought forward against an atheist to prove the existence of God.