"popular opinion" sometime is wrong.
See Descartes' Epistemology : Descartes established the "clear and distinct" criteria for truthfulness of ideas.
Descartes was not a skeptic: he adopted a "methodical doubt", i.e. assumed as a method of inquiry that every opinion that is not certain must be scrutinized.
In this way, he arrived at the well-known Cogito ergo sum conclusion : even if "there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No: if I convinced myself of something then I certainly existed." (See Med, III,1)
From this, he moves on to the "idea of God"; see Med, III, 17-22:
But among these my ideas, besides that which represents myself, respecting which there can be here no difficulty, there is one that represents a God; [...]
There only remains, therefore, the idea of God, in which I must consider whether there is anything that cannot be supposed to originate with myself. By the name God, I understand a substance infinite, [eternal, immutable], independent, all-knowing, all-powerful, and by which I myself, and every other thing that exists, if any such there be, were created. But these properties are so great and excellent, that the more attentively I consider them the less I feel persuaded that the idea I have of them owes its origin to myself alone. And thus it is absolutely necessary to conclude, from all that I have before said, that God exists