"But this does not prevent us from believing matters that have been divinely revealed as being more certain than our surest knowledge, since belief in these things, as all faith in obscure matters, is an action not of our intelligence, but of our will."
Descartes vote of confidence in the trustworthiness of Divine Revelation is worth noting, but it is a remnant of the medieval philosophers views of scripture, who claimed that Divine Revelation was the only authority to which we could appeal with absolute certainty. His claim that belief in the propositions of Divine Revelation is an act of the will is also in accord with Augustine, and Aquinas's teachings that belief is a voluntary act. However, it seems strange that he would deny that it is an act of the intellect. There does not seem to be any warrant for such a denial, except, perhaps, that for Descartes an act of the intellect could possibly be construed as either intuition or inference, in which case neither of these correspond to belief. However, it seems that belief is an act, not only of the will, but also of the intellect, as the will must accept a certain proposition, which is received by the intellect.
Whatever the case Descartes is certainly right to claim that Divine Revelation is, above all knowledge, most worthy of our trust.
Rene Descartes, "Rules for the Direction of the Mind," in The Philosophical Works of Descartes, trans. by Elizabeth S. Haldane and G. R. T. Ross (1911; repr., Cambridge: Cambridge University Press, 1984), 1:8.