Which two lines from john milton's paradise lost provide clues to his purpose in writing the epic poem?

I may assert th' Eternal Providence, 
And justifie the wayes of God to men. 

In these two lines the author tells the reader what his purpose is. 
He is invoking the Holy Spirit to help and guide him, and that is why he believes his poem is better than anything else written by any other classical poet. With these two lines he is justifying God's plans to have Adam and Eve commit the Original Sin, he believe it was all a plan. He also believe that his poem deals with subjects way more important than this one. 
Milton was an ambitious writer, wanting his poem to be the first English epic poem.