Sure, Milton wrote a LOT of poetry. But he also wrote a lot concerning the idea of liberty in a society that believed in Christianity. Here's a list of his works, courtesy of Christ's College Milton website.
Milton was a product of his time—a time that included the English Civil War and the regicide. Milton, very tied to republican ideologies, was an active member of the Commonwealth. According to scholar Rosanna Cox, "Milton's texts and ideas," in the context of the time period, wrote about what "constituted liberty and the terms on which it could exist."
But goodness, it's just hard to keep people free, what with the ease of going back to oppressive systems.
Even Milton's poetry exhibits his political ideals. His Samson Agonistes criticizes those who go back to old forms of government because it's easier to accept tyranny than build up a government that supports freedom. According to Cox, "Milton holds up a mirror to the nation and finds it wanting in fortitude, virtue and political agency; which is particularly poignant considering his hopes and his committed endeavors to the task of urging, cajoling, and hectoring the nation to fitness for their political destiny." Much of Milton's work hinges on the belief that without Republicanism, the people couldn't be free. Some of the things he wrote about were education, divorce, religious reform, censorship, and revolution.
Socrates, Thomas Jefferson, and John Milton speaking of liberty.
Milton was well-known and his works widely-read. His ideas and beliefs would help to shape a nation, and put him in jail for a time, too. The question I have is this: are his ideals sustainable in practice? People throughout history have shown that they are incapable of moderating their behavior and government powers. It is arguable that he doubted his ideal government could ever be implemented. While humanistic ideas melded with Christian principles may seem great, instituting that over an entire nation would be difficult.
Cox, Roseanne. "John Milton's Politics, Republicanism and the Terms of Liberty." Literature Compass 6.4 (2007): Blackwell Publishing Ltd.