As a Puritan, how does Milton employ classical epic convention in his invocation in Paradise Lost?
It is well worth considering how Milton in his invocation in Book 1 of this epic classic both upholds the conventions of invocations in other epics but also subverts them and brings his own Puritan world view into what he writes about. This invocation of course bears many similarities to the invocations in other epic classics such as Homer's The Iliad and The Odyssey. Such works appeal to a muse to help the poet achieve their purpose and also use elevated and grandiose language in their invocation. In addition, the invocation indicates that this classic starts in media res, or in the middle of things, as other epics do as well.
What is different is that Milton brings to his writing a Christian sensibility that springs directly from his Puritan background. Whereas other epics are pagan in nature, focusing on the relations between a pantheon of Gods who war against each other and use humans as their toys to further their own purposes, Milton states in his invocation that his epic classic is written to do nothing less than to "justify the ways of God to man." The invocation is full of Biblical imagery and allusions that support this purpose:
Of Mans First Disobedience, and the Fruit
Of that Forbidden Tree, whose mortal tast
Brought Death into the World, and all our woe,
With loss of Eden, till one greater Man
Restore us, and regain the blissful Seat,
Sing Heav'nly Muse, that on the secret top
Of Oreb, or of Sinai, didst inspire
That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
Rose out of Chaos:
You can count the references to important Biblical characters and themes, and in order to answer your question you would do well to compare this invocation with the invocations of other epic poems in order to explore further the distinctly Puritan nature of this opening.