Calvin claims that there is an awareness or sense of God (sensus divinitatis) implanted in all people by nature. Belief in God is universal according to Calvin. The context of this universally distributed belief is rather minimal: there is a God, He is the Creator, and that He ought to be worshipped.

Background: Cicero, De Natura Deorum (in which Cicero makes roughly the same point)

1. Simple Observation: Belief in God seems to be Universal
2. The diversity of religious practices and beliefs all presuppose some basic conception of divinity or a Supreme power in the Universe.
3. Those who are impious and object to the existence of God nonetheless have in their minds an idea of such a being, so even they are aware of God.

The function of such an awareness of God is to render humans without excuse before God. They cannot plead ignorance when it comes to divine judgment on their lives. Hence, this knowledge of God possessed by people by nature is closely related to distinctly moral and theological concerns.

1. It might be objected that certain people have invented religion in order to control the masses or common folk. Calvin thinks that such attempts would not be effective unless people already possessed some awareness of God. How else could religion have power over people? There must be some antecedent sympathy or seriousness about religious matters, if leaders or others are able to use it to manipulate people.
2. It might also be objected that some people (i.e., atheists and agnostics) do not believe in a God of any sort. So belief in God is not universal. Calvin has at least three responses here.

1. First, perhaps some do lack such belief and knowledge, but perhaps this is the result of their doing and does not represent their original condition. Calvin says that it is possible to affect adversely this knowledge, perhaps not just its intensity but also its very presence. How? Acts of sin, especially great wickedness, can deaden the conscience and remove God from our awareness. Such individuals would still be morally accountable because originally they had such knowledge of God, perhaps as a young child, but then lost it as the result of choices freely made.
2. Another point suggested by Calvin and compatible with the prior point is that lack of belief in God may only be a temporary thing. There is a universal awareness of God, even if not everyone has such an awareness of God at all times.
3. Third, though not explicitly addressed by Calvin, is the possibility that some people do believe in God but either do not believe that they do or believe that they do not believe in God. Perhaps there is a kind of self-deception here. This response depends, contra Descartes, on the mind not being fully transparent to itself. There could be a subconscious realm where religious beliefs can reside though we are not conscious of them. So people might know God without knowing that they know God. This suggestion seems plausible given our understanding of the human psyche since the 19^th century (e.g., Freudianism). Consider also the fact that we are only conscious of a very limited number of our beliefs at a given time. The rest are non-occurrent and must be brought to consciousness. Usually this is easily, but sometimes we forget things or just can't recall them at will. This can be drastic in cases like amnesia. Normally we don’t say that people who once consciously believed something, then not conscious of it but later become conscious of it again did not hold the belief in the intervening period of unconsciousness.

In addition to the sense of divinity within (Institutes, Book 1, chapter 3), Calvin recognizes that there is an external witness to God in creation, in the physical, visible world (Institutes, Book 1, chapter 5).

One of the main questions surrounding these passages is whether Calvin is presenting something like an argument for God's existence. More generally, is Calvin claiming that human persons infer God's existence from observations of the physical world?

Clearly Calvin is not offering the sort of extended syllogistic reasoning found in Aquinas and other medieval theologians. However, it certainly seems that he thinks that some of God's attributes, such as wisdom and power, are "displayed" or "revealed" in creation. But this presupposes the kind of causal principle we find in Aquinas, namely that effects resemble their causes. Owing to Calvin's rhetorical style of argument, we should not expect him to formulate formal arguments like Aquinas. But it looks like there is at least a hint of some inferential knowledge of God, which, like Aquinas' arguments, takes as its starting point observations of the physical world. (For an explanation and defense of inferential natural knowledge of God in Calvin account, see my paper "The Prospects for Mediate Natural Theology in John Calvin," Religious Studies (March 1995)).

It might be objected that inferential knowledge of God is superfluous since all people already believe in God by way of the Sensus Divinitatis.

The knowledge delivered by the sensus divinitatis is rather minimal in content. Calvin says nothing about it delivering or producing beliefs about God being wise, good, or powerful. The attributes of God seem to be the sort of thing that is discovered through observations of the physical world, not the operation of the sensus divinitatis by itself. So perhaps the sensus divinitatis and external witness each deliver different sorts of beliefs, but are intended to work together to produce a more complete knowledge of God as the creator, with attributes of goodness and wisdom. (Reformed theologians subsequent to Calvin typically distinguished between these as two independent but mutually supportive modes of knowing God by reason).

As we shall see later in the course, Alvin Plantinga maintains that the sensus divinitatis can be interpreted a disposition to form certain religious beliefs, and it is triggered by the kinds of circumstances mentioned by Calvin as parts of the external witness. This does not involve drawing an inference from observations of nature, but rather an automatic cognitive response to the experience of the starry night sky, the complexity of the human body, etc. According to Plantinga, the sensus divinitatis and the external witness are two aspects to one process by which human reason comes to know truths about God as creator. (See my paper, "The Prospects for Mediate Natural Theology in John Calvin.")

Calvin distinguishes between the knowledge of God as creator and the knowledge of God as redeemer. The former is accessible to human reason and constitutes our natural knowledge of God; the latter is not accessible to human reason but must be revealed by God and is believed, not because of reason, but because of what Calvin called the internal testimony of the Holy Spirit.

The knowledge of God as redeemer includes as central the truths about the person and work of Jesus Christ. Since such truth is found only in Scripture, the internal testimony of the Spirit is closely connected to belief that the Bible is God's Word. God's Word, the Bible, at once corrects mistaken beliefs about God as creator and clarifies the knowledge of God as creator had by reason. Furthermore it reveals many new truths about God, specifically in relation to salvation. For Calvin, Scripture clarifies, corrects, and augments the natural knowledge of God. (Compare this with Aquinas' distinction between the preambles and articles of faith and the relationship between faith and reason).

Calvin thinks that salvation involves the indwelling of God's Holy Spirit in the person, this indwelling imparts spiritual sight and enables people to see the truth of Christianity, specifically the truth of the Bible as God's Word. Calvin says that for believers the Bible is "self-authenticating." Calvin rejects the idea that believers need to prove that the Bible is God's Word, though he permits and actually himself engages a use of argument for the purposes of "useful confirmations." Calvin is clear that faith is first and argument second, but faith is not based on argumentation.