How do you make an allegory not suck? Treat it as a story with characters, textures, ambiance, and plot twists. That may carry you off the beaten track of moralizing, but at least you shall deserve a legacy as literature rather than as a virtuous bore.
And that’s the kind of deal with the Devil Milton is known for, making Satan a much more interesting character than God, but it’s also one that Edmund Spenser did earlier in the Faerie Queene, by making his virtuous knights fight against all sorts of evil temptations.
Weaving together the Arthurian tradition, medieval moral allegory, the poetics of Italian romances, in a language that was, even for its era, archaic-sounding and Latinate, The Faerie Queene is a thicket, albeit a rich and colourful one. French speakers may find themselves surprisingly at home with the language, given its use of Latin syntax (placing for example adjectives after nouns) and French-inspired vocabulary.
The correspondance allegory of the Grail cycle is here enlivened by the evocative descriptions of setting and characters, and the plot has learned the lessons of suspense and tension from the Orlando Furioso or the Gerusalemme. We may not reach the philosophical paradoxes of Shakespeare, but we are meant to be entertained and taught.
Modern readers often turn to the Faerie Queene on the strength of its plot, often by way of C.S. Lewis’s defence of it, but I found its greatest delight in its language. Many good yarns with enchanters, temptresses, magical islands, and mystic overtones can be found, but few have a range of expression as dense as Spenser’s, as if he mustered Petrarch’s lyrical powers for epic purposes.
Spenser, Edmund. The Faerie Queene. In 8 vols. General editor Abraham Stoll. Cambridge, MA: Hackett. 2008. ISBN 978-0-87220-941-1.