Longfellow builds a whole argument out of one repeated line. Every stanza ends on "peace on earth, good-will to men," and the interest lies in watching what happens to that line as the poem goes on. At first it is a carol, "wild and sweet." Then it becomes a worldwide chorus rolling through "the belfries of all Christendom." Then cannon drown it, then it turns into a ruin of households, then a slogan that hatred mocks, and finally a promise. The words never change, but the weight on them does.
The story is simple. A listener hears Christmas bells, imagines them ringing across the world, and is jolted by gunfire "in the South" that smothers the music. He despairs and says aloud that "There is no peace on earth." The bells reply. The poem never names a war, but the cannon and the "continent" whose hearth-stones are split suggest a nation fighting itself. Its central idea is that hope in peace has to survive the evidence against it. The poem doesn't pretend the evidence away. It gives despair a full stanza and lets it speak in the speaker's own voice before anything contradicts it.
The form carries much of this. Each stanza has five lines rhymed AABBC, and the C line is always the refrain, so "men" never finds a rhyme-partner inside its own stanza. It is left hanging like a bell stroke, with the echo coming from the next stanza. The rhythm helps. Two four-beat lines open each stanza ("I heard the bells on Christmas Day / Their old familiar carols play"). Then two short, two-beat lines follow ("A voice, a chime / A chant sublime"), which tap out like small hammer-blows on a bell, before the long refrain swings back.
The sound of the language shifts with the mood. The peaceful stanzas are full of soft, flowing sounds: "ringing, singing," "rolled along / The unbroken song." The word "unbroken" is a quiet setup, because the next stanza breaks the song. When the cannon arrive, the poem hardens. "Each black accursed mouth" packs three heavy words into a row, and the rhymes "mouth / South" and "sound / drowned" boom with an open, thudding vowel. Calling the cannon a "mouth" is clever, since it makes a rival voice to the bells, one that shouts the carol down instead of singing it.
Longfellow also plays with scale. In the third stanza the world itself "revolved from night to day," as if the planet were a bell turning in its frame. Two stanzas later the view shrinks to "hearth-stones," the most ordinary household object imaginable, now split by an "earthquake" the size of "a continent." War is imagined at once as a planetary disaster and as something that wrecks living rooms. The grammar of the refrain bends to this idea as well. The households are "born / Of peace on earth," so the sung phrase becomes their parent. In the despair stanza, hate "mocks the song / Of peace on earth," and the refrain turns into the victim of a sentence.
The last stanza is where the poem earns its ending. The bells now peal "more loud and deep" and speak directly, in an older, scriptural register: "God is not dead; nor doth he sleep!" The tense changes too. Until now the poem has been in the past, with "heard," "thought," "thundered." The bells answer in the future: "The Wrong shall fail, / The Right prevail." And the refrain changes for the first time, from "Of peace on earth" to "With peace on earth." Peace is no longer only the subject of a song. It arrives as the result of the Right prevailing.
That is what makes the poem worth reading beyond the season. A reader might ask whether this confidence is earned, since the bells assert rather than prove. But the poem is honest about the difficulty. The hope doesn't come from ignoring the cannon. It comes after them, louder than before, and that is what gives the final line its force.