In his Institutes of the Christian Religion, Calvin argues that God makes himself evident in both nature and man himself. God reveals himself so clearly that man is without excuse for not knowing him. However, man suppresses the truth about God and thus needs special revelation to truly understand God.
In speaking this way, Calvin explains knowledge in one realm – the heavenly realm. But there are two realms. Here he explains the difference between the heavenly realm and the earthly realm:
It may therefore be proper, in order to make it more manifest how far our ability extends in regard to these two classes of objects, to draw a distinction between them. The distinction is, that we have one kind of intelligence of earthly things, and another of heavenly things. By earthly things, I mean those which relate not to God and his kingdom, to true righteousness and future blessedness, but have some connection with the present life, and are in a manner confined within its boundaries. By heavenly things, I mean the pure knowledge of God, the method of true righteousness, and the mysteries of the heavenly kingdom. To the former belong matters of policy and economy, all mechanical arts and liberal studies. To the latter (as to which, see the eighteenth and following sections) belong the knowledge of God and of his will, and the means of framing the life in accordance with them. (2.2.13)
Regarding men thinking on “earthly things,” Calvin sees a kind of universal reason regarding civil order:
…Hence it is that every individual understands how human societies must he regulated by laws, and also is able to comprehend the principles of those laws. Hence the universal agreement in regard to such subjects, both among nations and individuals, the seeds of them being implanted in the breasts of all without a teacher or lawgiver. (2.2.13)
…some principle of civil order is impressed on all. And this is ample proof, that, in regard to the constitution of the present life, no man is devoid of the light of reason. (2.2.13)
By the light of reason men perceive more than just civil order. It can be the source of admirable work in the liberal arts:
…Therefore, in reading profane authors, the admirable light of truth displayed in them should remind us, that the human mind, however much fallen and perverted from its original integrity, is still adorned and invested with admirable gifts from its Creator. (2.2.15)
…Shall we say that the philosophers, in their exquisite researches and skilful description of nature, were blind? Shall we deny the possession of intellect to those who drew up rules for discourse, and taught us to speak in accordance with reason? Shall we say that those who, by the cultivation of the medical art, expended their industry in our behalf were only raving? What shall we say of the mathematical sciences? Shall we deem them to be the dreams of madmen? Nay, we cannot read the writings of the ancients on these subjects without the highest admiration; an admiration which their excellence will not allow us to withhold. (2.2.15)
It may seem painfully obvious that the work of non-Christians in “earthly things” can be not only legitimate but, moreover, praiseworthy. If that’s what you’re thinking, good for you. But, believe it or not, there are some who doubt the existence of the light of reason and disparage whatever is not explicitly grounded in religion. Well, they will do what they do and say what they say, but they can’t say Calvin is in their corner.