Preface Summary

In this Monody the Author bewails a learned Friend, unfortunately drown'd in his Passage from Chester on the Irish Seas, 1637. And by occasion fortels the ruine of our corrupted Clergy then in their height.

• Milton included this preface in the 1645 reprint of this poem as a part of his collection Poems of Mr. John Milton.
• Milton was quite the political guy, and he was no fan of the Church, or the British monarchy for that matter.
• When this poem was first written, the British monarchy was very much in power politically, while the Church of England held sway over the religion of the country. Milton detested the hierarchical power of the Church, because he believed that the corrupt clergy would interfere with people's devotion to God.
• Later, in 1645, when his collection of poems was published, political and religious upheaval in England had begun to carve some chinks in the armor of the British Monarchy and the clergy, which was pleasing to Milton. Perhaps that's why he included this brief preface. He's saying "I told you so" to those corrupt clergy members who had threatened the religious freedoms of British citizens.
• This preface gives us a lens through which to read the poem, particularly the section in which St. Peter rails against good-for-nothing shepherds who don't properly tend to their sheep. Knowing Milton's religious and political beliefs, we can see these shepherds as a metaphor for pastors and clergymen run amok, and St. Peter's assurance that they will pay the ultimate price in the afterlife as a metaphor for Milton's belief (which was proven right) that the Church won't have such power forever.
• Oh, and by the way, a monody is a type of poem in which the poet laments the death of a friend or fellow poet. Sounds about right.