How is Book I and the Prefatory Matter of Paradise Lost radical?
It is important in responding to this question to be clear about the way in which Milton both writes his epic in line with previous epics by Homer and Virgil, but at the same time makes obvious changes that surpass the model of an "epic" and indicate his purpose for writing. It was typical for epic poems to begin with a dedication to a muse for inspiration and help in writing the poem, yet Milton distinguishes his work and makes it radical through an appeal to the Holy Spirit and an aim that appears to be incredibly arrogant on the one hand, and yet this appeal to the Holy Spirit is made with real humility:
And chiefly thou, O Spirit, that dost prefer
Before all temples th'upright heart and pure,
Instruct me, for thou know'st; thou from the first
Wast present, and with mighty wings outspread
Dovelike sat'st brooding on the vast abyss,
And mad'st it pregnant: what in me is dark
Illumine; what is low, raise and support;
That to the height of this great argument
I may assert Eternal Providence
And justify the ways of God to men.
Note Milton's hope that in this poem he may do nothing less than "justify the ways of God to men." This clearly is an extremely radical hope as he hopes he can explain God's actions and "justify" them to his human audience. In terms of its scope, therefore, this work is incredibly radical in what it seeks to do, and it is likewise radical in form, as it both follows the set conventions of the epic whilst also deviating from them to indicate its own specific purpose and context as Milton takes the greatest story of them all, the story of humanity's creation and existence, and seeks to explore it further for God's glory. Given this, it is hard not to be struck by the bold and daring nature of this epic, even as Milton himself humbly acknowledges that he needs help to be successful in his goal.