There are several moments of genius in Calvin's Institutes.
The first is in the very first chapter when he notes...
"Our wisdom, in so far as it ought to be deemed true and solid Wisdom, consists almost entirely of two parts: the knowledge of God and of ourselves.
But as these are connected together by many ties, it is not easy to determine which of the two precedes and gives birth to the other."
Theology is based entirely on both anthropology and theology proper - knowledge of humanity and knowledge of the Trinity.
Calvin is aware that it is equally good to start with anthropology... and we might wonder how much better (or at least different) might Reformed theology have been if Calvin had taken that other road and begun with the humanity of Jesus first.
However, Calvin attempted to address the knowledge of God first... and set in stone a set of assumptions and methods that have been locked into Reformed theology ever since.
(Schleiermacher chose the other route - and Warfield contemplates that possibility... )
In the LORD Jesus Christ both humanity and divinity are equally ultimate and perfectly united.
So long as HE is the focussing lens, it is safe to take either route - beginning with humanity or trying to begin with divinity.
The problem is that if we try to BEGIN with divinity in abstraction, HOW do we do that?
If the only genuine knowledge of God is found in the God-Man Jesus Christ (or perhaps in the pre-incarnate Son), then how could we start with any idea of divinity that is not already given to us in Jesus - with the incarnation achieved or anticipated?
The only "divinity" presented to us in the Bible is in the person of God the Son.
The strategy of what has come to be known as "classical theism" is to accept the philosophically traditional idea of divinity from Plato & Aristotle as a suitable starting point - and then to work back from that to the Bible.
So, the Neo-Platonist tradition of the 3rd century defined "god" as "The One" and saw divinity as "a necessary, self-existent, simple, omnipotent, omniscient, omnipresent, perfectly good, immutable, impassible and eternal essence".
There was a significant shift in Mediterranean Christianity when leading Christian thinkers accepted this definition on philosophical grounds and then tried to argue that this definition of divinity was at least compatible with the Bible's teaching - and some argued that it was directly taught by the Bible.
HOWEVER, if divinity is defined from outside the Bible and THEN we attempt to integrate this into the Bible, there is a very real problem that we are working with an alien conception of divinity... and however hard we try to integrate this into a Biblical theology, there is something twisted and even corrupt all the way.
Athanasius had the idea that the only system that can contain Jesus MUST start with Him - and so classical theism, for all its successes, is constantly trying to fit Jesus into a system that does not start with Him.
What if Calvin had chosen the other route and begun with anthropology?
Would he have simply adopted a pagan or "common sense" view of humanity... or would he have been able to develop a view of humanity defined only by Jesus of Nazareth?
AND, from such a view of humanity, would Calvin have then started with a vision of divinity that began with Jesus?