John Calvin on the Ethical Nature of Belief in God

Calvin affirms that all men have a knowledge of God, but that they do not respond properly to this knowledge. The sinful nature, which effects the intellect, is the source of unbelief, not a lack of information. Unbelievers know God exists. This reality also takes the gusto out of the argument of universalists or sycretists who argue their position based upon a vague notion of God found in various religions. Calvin writes:

Nevertheless, it is one thing to feel that God as our Maker supports us by his power, governs us by his providence, nourishes us by his goodness, and attends us with all sorts of blessings-and another thing to embrace the grace of reconciliation offered to us in Christ.

Further, he states,

Moreover, although our mind cannot apprehend God without rendering some honor to him, it will not suffice simply to hold that there is One whom all ought to honor and adore, unless we are also persuaded that he is the fountain of every good, and that we must seeking nothing elsewhere than in him [...] (Institutes Book I. Ch. II.i)

According to Calvin (and Paul in Romans 1), man does not have an intellectual problem, he has a moral problem with God. He knows who God is, as even the demons know God and tremble (James 2:19). We should not be surprised then that we see the knowledge of God in all men, and that often they may give themselves over to their better knowledge as they seek a benefit from doing so (ie to appease conscience, earn blessings, or be "moral"). The problem according to Scripture, as Calvin points out, is that men are "holding back" or suppressing the truth. They will not fully commit themselves to God because they do not trust in the goodness of God. Their unbelief is rooted in an implicit slander against God. It is founded in ungodliness and is strengthened by their unrighteousness. Calvin elaborates:

For until men recognize that they owe everything to God, that they are nourished by his fatherly care, that he is the Author of their every good, that they should seek nothing beyond him-they will never yield him willing service. Nay, unless they establish their complete happiness in him, they will never give themselves truly and sincerely to him. (Ibid)

If you examine the narrative of the fall, in Genesis 3, it is the goodness of God that Satan attacks in tempting Eve to turn from him and try and become god herself. Indeed, this noetic (or mental) effect of sin is implanted in all of us, unless the Holy Spirit regenerates us. That is why we grow in our sanctification as we come to find our joy in the goodness and grace of God. To hear this fleshed out further, check out this sermon on Genesis 3.