Descartes may have used God to prove reason works, and reason to prove God exists
To escape doubting literally everything, Descartes argued his own clear and distinct perceptions must be true because a non-deceiving God guarantees them. But his proof that God exists itself relies on trusting his own clear and distinct perceptions — the very thing that proof was supposed to secure. Critics from Antoine Arnauld onward have called this the 'Cartesian circle': reasoning that quietly assumes the reliability it's trying to establish.