Book I is called "The Knowledge of God the Creator." Chapter I of Book I is: "The Knowledge of God and That of Ourselves Are Connected. How They Are Interrelated."
I suppose theology must have a place to start. The Institutes begin with human existence, but not human existence in abstracto, but human existence as it can only be lived: in indissoluble connection with the God who (as Karl Barth would say) does not will to be God without man. We have the God who is free to exist happily apart from man, but who wills from eternity not to exist in this way, and we have man who can never exist apart from God. This reality--God and man--is where Calvin begins his institutes.
There is something of an insoluble dialectic at work here that needn't be solved. Man needs knowledge of himself and knowledge of God, but how can he come to either? Apart from knowledge of himself, man can have no knowledge of God. (Note: the end, the goal is knowledge of God!) This is Calvin's presupposition which doesn't seem to be explained in any detail. It is merely stated. Now, according to Calvin there are two ways in which we may find a knowledge of God through self-knowledge. First, we might be able to look upon our endowments and, "led as rivulets to the spring itself," ascend to the knowledge of the God who gives these good gifts. Without much deliberation on this prospect, Calvin abandons this tack, in order to say that the real way to God is through an embrace of our poverty and ruin. Only when we see our desperate situation are we led to seek God.
The situation is not so simple, however, because there is no knowledge of self without the prior knowledge of God! As fallen, sinful people we run around in hypocrisy and we take our relative (!) goodness to be absolute goodness, declaring ourselves righteous and holy when we have no vantage point from which we can accurately make such declarations. Until we see God in his holiness, there is no way that we can see ourselves in our sinfulness. In Calvin's words:
"So it happens in estimating our spiritual goods. As long as we do not look beyond the earth, being quite content with our own righteousness, wisdom, and virtue, we flatter ourselves most sweetly, and fancy ourselves all but demigods. Suppose we but once begin to raise our thoughts to God, and to ponder his nature, and how completely perfect are his righteousness, wisdom, and power--the straightedge to which we must be shaped. Then, what masquerading earlier as righteousness was pleasing in us will soon grow filthy in its consummate wickedness. What wonderfully impressed us under the name of wisdom will stink in its very foolishness. What wore the face of power will prove itself the most miserable weakness. That is, what in us seems perfection itself corresponds ill to the purity of God."
The situation makes perfect sense: we can't have knowledge of God without knowledge of self, but we also can't have knowledge of self without knowledge of God. (Keep in mind, I don't think Calvin is speaking of some type of ontological knowledge here. He must be using the word "knowledge" in a much deeper, richer sense that this). Yes, it makes logical sense. But this doesn't answer the question: how is it possible?
The answer is this: it happens. It is real. It is possible because it happens. As those in more modern times have said, "what is actual is possible." The biblical witness shows us the reality of those who have been confronted by God and have come to knowledge of themselves in their poverty and God in His glory. Calvin speaks of Abraham, Elijah, Job, and Isaiah as the archetypes of this happening. It is the insoluble dialectic (pardon the term, I know it can be fraught with all kinds of weird meanings) that is the context of this happening of the knowledge of God. This happening is "revelation."
And so revelation comes upon man. This is the presupposition (nay, the reality) that theology must begin from. And this revelation is not information (primarily), it is the presence of the Living God. It has ramifications for man.
"Hence that dread and wonder with which Scripture commonly represents the saints as stricken and overcome whenever they felt the presence of God."