There are at least two significant interactions with theological voluntarism in Descartes account of certitude, but it’s unclear whether he’s trying to refute them or incorporate them into his thought. I’ll give a brief account of voluntarism here and talk about how it shows up in the Meditations later.
Voluntarism means more than one thing, I’m here taking it as the thesis that, given that God is omnipotent, anything logically possible in creation becomes really possible. The upshot of this is that anything we can imagine being the case could really be the case. Any coherent story, or even any story that plays by a plausible set of logical rules (which is usually, though perhaps not necessarily, understood as any account escapes formal logical contradiction) becomes a live possibility for how things actually could be.
One way of avoiding the conclusions of voluntarism so construed was to make the divine will posterior to the divine ideas, and to identify the divine ideas with the essences of the things around us, where “essence” was understood to be a stable, unchanging, eternal principle that thus grounded the possibility of certitude and science. To my mind, there are two difficulties that the account has to overcome: First, “essence” is medieval shorthand for Aristotle’s “what it is”, and given the mutable character of creation (and especially Aristotle’s introduction of prime matter into the essences of cosmic things), it is not obvious how a created essence can be the principle of a scientific certitude; second it is not clear why one should identify the divine ideas with essences as understood by the Greek tradition as filtered though and developed by the Medievals. Taking Scripture seriously, for example, suggests that the divine ideas are less about essences and more about concrete, unique historical facts. The Medieval “Book of creation” is not a story of hierarchically ordered essences which the human mind is called to read off of nature and arrange in a universal system, but simply facts that become universal and unchanging in the human mind but have no basis for this sort of existence elsewhere.