The Great Fact

I have lately been reading Biblical Theology by the Reformed writer Geerhardus Vos.  This is an interesting text that is divided into three parts: the Mosaic epoch of revelation, the prophetic epoch of revelation, and the New Testament.  As I was reading in the third part, I came across a striking phrase:

"...He is the great fact to be expounded."

 From Wikimedia Commons, Geerhardus Vos

Who is this "great fact"?  Why do we regard this person as a "fact"?  Moreover, why is this fact something to be expounded, something to be reckoned with, a kind of cornerstone?  I believe this person is the fact that enables all meaning.