The answer to this question can be found in Book 11 of this amazing classic, which is when God sends the angel Michael down to Adam to both expel Adam and Eve from the garden but also to explain why they are being expelled and what the future of mankind will look like. What is important to note is that, in the series of visions that Michael shows Adam, the depths of deprativty into which mankind will sink is balanced at each stage by at least one faithful human who will be used to save the rest of mankind. Note, for example, how Michael shows presents Noah:

One Man except, the onely Son of light
In a dark Age, against example good,
Against allurement, custom, and a World
Offended; fearless of reproach and scorn,
Or violence, hee of wicked wayes
Shall them admonish, and before them set
The paths of righteousness, how much more safe,
And full of peace, denouncing wrauth to come
On thir impenitence; and shall returne
Of them derided, but of God observd
The one just Man alive...

Michael tells Adam this future of his race in order to fulfill the original aim of the poem as set out in Book 1, to "justify the ways of God to man." Even though the future of mankind that Michael shows to Adam presents man as being driven inexorably by their sinful desires and passions, it is clear that God has a plan to "before them set/The paths of righteousness" at each stage throughout history. Noah, of course, like other important figures in the Biblical story, prefigures the ultimate saviour of mankind, Jesus Christ. Michael therefore both shows Adam the natural sinful inclination of humans and how it will impact his descendants, but he also encourages him that God will not abandon his creation and that he will ultimately use "one just man" to enact the salvation of his fallen humans.