Nature does not give us knowledge of what God is

"The mystical theology of the Egyptians also shows that they all sedulously endeavoured to preserve the appearance of reason in the midst of their folly. And any thing apparently probable might at first sight, perhaps, deceive the simple and incautious; but there never was any human invention by which religion was not basely corrupted. And this confused diversity emboldened the Epicureans, and other gross despisers of piety, to reject all idea of God. For, seeing the wisest of men contending with each other for contrary opinions, they hesitated not, from their discussions, and from the frivolous and absurd doctrines maintained by the different parties, to infer, that it was vain and foolish for men to torment themselves with investigations concerning God, who does not exist. And this they thought they might do with impunity, supposing that a compendious denial of any God at all would be better than feigning uncertain gods, and thereby occasioning endless controversies. they reason very ignorantly, or rather endeavor to conceal their own impiety behind the ignorance of men, which not at all justifies any encroachment on God. But from the general confession, that there is no subject productive of so many dissensions among the learned as well as the unlearned, it is inferred, that the minds of men, which err so much in investigations concerning God, are extremely blind and stupid in celestial mysteries. Others commend the answers of Simonides, who, being asked by Hiero the Tyrant what God was, requested a day to consider it. When the tyrant, the next day, repeated the inquiry, he begged to be allowed two days longer; and, having often doubled the number of days, at length answered, 'The longer I consider the subject, the more obscure it appears to me.' He prudently suspended his opinion on a subject so obscure to him; yet this shows that men, who are taught only by nature, have no certain, sound, or distinct knowledge, but are confined to confused principles, so that they worship an unknown God." - John Calvin, The Institues, I. V. XII.

We are stupid apart and derive no advantage of the testimony of God in nature

"But, notwithstanding the clear representations given by God in the mirror of his works, both of himself and of his everlasting dominion, such is our stupidity, that, always inattentive to these obvious testimonies, we derive no advantage from them. For, with regard to the structure and very beautiful organization of the world, how few of us are there, who, when lifting up their eyes to heaven, or looking round on the various regions of the earth, direct their minds to the remembrance of the Creator, and do not rather content themselves with a view of his works, to the total neglect of their Author! And with respect to those things that daily happen out of the ordinary course of nature, is it not the general opinion, that men are rolled and whirled about by the blind temerity of fortune, rather than governed by the providence of God? Or if, by the guidance and direction of these things, we are ever driven (as all men must sometimes be) to the consideration of a God, yet, when we have rashly conceived an idea of some deity, we soon slide into our own carnal dreams, or depraved inventions, corrupting by our vanity the purity of divine truth. We differ from one another, in that each individual imbibes some peculiarity of error; but we perfectly agree in a universal departure from the one true God, to preposterous trifles. This disease affects, not only the vulgar and ignorant, but the most eminent, and those who, in other things, discover peculiar sagacity." - John Calvin, The Institutes, I.V.XI