The Ninth does away with many of those conventions, retaining some elements of Classical style while blazing a path for the Romantic period that followed. Unfurling over 20 minutes, its “Ode” looks like recognizable forms while resisting categorization. With a remarkably free spirit, the music comments on the poem’s text and spins fantasies from it, as the historian Harvey Sachs has written.

Those fantasies most closely resemble a theme and variations, a musical form in which a melody is put through a series of stylistic transformations; the melody, in this case, is the famous song we know as the “Ode.” After it is first presented, Beethoven writes a topsy-turvy, bacchic version of it. Instead of one note per syllable, there are now two, rising and falling, as if performed by a drunken singer incapable of steadiness. But at the mention of God, the music becomes awe-inspiringly mighty, with beaming focus. It sounds like an ending, but after a pause, the instruments of a military band enter with a Turkish march: a non sequitur with a touch of kitsch, perhaps, or parody.

Or maybe it’s a harbinger of war, because next, a battle begins: one of keys, and by proxy the spirit, represented by a fugue, from which the radiant key of D emerges with the biggest treatment yet of the “Ode” melody. The only thing left, after all that, is to celebrate.

With the sound of sacred music, the tenors and basses, doubled by deep-voiced instruments in the orchestra, enter with the phrase “Be embraced, you millions!” In the next section, Beethoven includes the word “devout” in his instructions to the musicians, and listeners can hear a move toward the divine, “beyond the starry canopy,” as Schiller describes the realm of “the Creator.” Celestial awe gives way to a double fugue of holy grandeur.

Once again, the piece seems to have reached a conclusion, but it keeps going. The vocal soloists enter, repeating text from the beginning that sets up a return to the line “All men become brothers” and the escalation of a proper coda. Beethoven brings back the military instruments, and the chorus sings the first line of the “Ode” at full power: “Joy, beauteous spark of divinity.” You can picture him almost warming up for a new section, but the orchestra manically, breathlessly brings down the curtain as if to shut down an argument that could go on forever.

Historically, a kind of argument did go on. Theist without being attached to any faith, as political as a “Coexist” bumper sticker, the “Ode” has been adopted across ideologies, with the music’s vision of Elysium changing in the eye of each beholder.