An epic poem, by definition, has no reference to religion or religious beliefs. Beowulf, about a pagan hero, is an epic poem. Paradise Lost, about conflicts between Christian spiritual forces, is an epic. The definition of an epic poem is that it is a long poem on a serious topic that has a story about a hero. Homer's Iliad is an epic. The confusion combining epic with Christianity is that Milton's Paradise Lost is often the only epic students read and study, and since it is written from a Christian view point and world view, it comes to be seen as defining the meaning of epic. But this is an error as the definition of epic, given above, is removed from Christianity or any religion, though the author's religious ideas may certainly be present within the epic.
The epic is appealing to everyone who loves the beauty of words. Surely that also includes non-Christians? Even people who don't necessary believe in the story of Adam and Eve and other Bible stories can appreciate them as literature...much like some people read the Koran, The Book of Mormon, Native American creation stories and other books/stories that are religious in nature simply for the story.
Epics also typically deal with a hero who represents a particular people and who embodies the very best of that society. The hero usually takes a trip, part of which requires him to go up against great odds and enemies of fantastic natures. With the help of supernatural means (gods, goddesses, angels, ghosts, magic, etc) the epic hero is able to defeat the enemy, but only after a dramatic struggle. Everyone enjoys a good story, and epics are always excellent reads.
The epic is universally appealing because it comprises the best values and ideals of the society in which it is written. I wouldn't say the epic is appealing to also non-Christians but to anyone who appreciates master poetry--great ideas with the right words.
Milton's Paradise Lost is a grand subject in which he tries to answer why God permits suffering. His style is both challenging and innovative, subjects often come after predicates, as in the very first line ends in line 16 and has to be inverted to obtain the correct meaning. The characters are ancient foes of God vs the Devil, and the story is interesting to anyone.