Yes — with one important distinction. You can demonstrate that God exists by reason alone, without appealing to faith or Scripture. What reason cannot do is compel anyone to love and worship Him; that final step belongs to grace and free will. But the existence of a First Cause — eternal, uncaused, the source of all being — is a conclusion reason can reach on its own. This is not one Catholic opinion among others; it is dogma. The First Vatican Council (1870) solemnly taught that God “can be known with certainty by the natural light of human reason from created things.”

St. Paul said as much at the very start: the reality of God is “clearly seen, being understood by the things that are made” (Romans 1:20), so that those who deny Him are left “inexcusable.” St. Thomas Aquinas later set the demonstration out as his famous Five Ways1 — the arguments from motion, causation, contingency, degrees of perfection, and the order of nature. Each begins from something undeniable about the world (things change; things are caused; things exist that need not have existed) and reasons to a necessary first principle on which all else depends. The modern versions — the Kalam cosmological argument, the argument from cosmic fine-tuning — reason in the same direction: from a feature of the world to the ground it requires.

What reason delivers is a Creator, not yet the Trinity — the God of the philosophers, not yet the Father of Jesus Christ. That fuller knowledge comes only through revelation. So the honest claim is modest and forceful at once: belief in God is not a leap against the evidence but a conclusion the evidence supports — and the burden is at least as heavy on anyone who insists the cause of everything is nothing at all. The full case for God, with the usual objections answered, is worth working through slowly.