God and time play crucial, intricately related roles in Descartes' project of grounding mathematical physics on metaphysical first principles. This naturally raises the perennial theological question of God's precise relation to time. I argue, against the strong current of recent commentary, that Descartes' God is fully temporal. This means that God's duration is successive, with parts ordered ‘before and after’, rather than permanent or ‘all at once’. My argument will underscore the seamless connection between Descartes' theology and his physics, and the degree to which he was prepared to depart from orthodoxy in the former in order to secure an a priori foundation for the latter. As Newton would later do, Descartes freed time from its traditional dependence on bodily motion and so removed an important barrier to making God temporal. Acting in time, God makes the physical world intelligible in a way He could not were He timeless.