If you actually had any familarity with Descartes and the question of God's existance, it would point you to Aristotle....
In Metaphysics, Aristotle develops a first science that studies being as being. The goal of this study is to discover the first cause or causes (Metaphysics -1). He concludes that there is an Unmoved Mover that is the only being capable of causing eternal motion and thus is the ultimate cause of the universe. Aristotle calls this Mover God (Ch.7). As a result, the first science is theology. Whatever explains substances explains all things, and for Aristotle, God or the divine explains the nature of substance by virtue of being the highest substance. If there were no substance, there would be nothing. Therefore, the science of divine being, theology, is the most universal science, prior to all other sciences, explaining everything by reference to the divine Unmoved Mover.
Descartes, although through a different methodology, ultimately proves the existence of this Aristotelian God, on the level of ideas, not beings in his Meditations on First Philosophy.
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Though Aristotle and Descartes took somewhat different approaches to discovering the first cause of the universe, they arrived at similar conclusions. First, that there is in fact a first cause to the universe, and second, that this cause can be nothing other than God. Aristotle proved the existence of a God that is the most complete being, eternal, and immovable. He reached this conclusion through an inquiry into substance. Descartes attempted to prove the existence of a perfect, eternal, infinite, and immovable God by borrowing Aristotelian concepts. However, he did so not through an inquiry into substance, which he was not particularly concerned with, but through his search for the cause of ideas. Ultimately, God is the cause of the universe for both Aristotle and Descartes. For both, God is necessary for existence.
Add in Newton's Laws of Motion, and it agrees with both Aristotle and Descartes views in that God is the first unmoved mover. In fact, it proves them in nothing less than cold hard logic and science. The rest is faith.