I'd like to clear up a few loose ends and objections that people have given me. This is an important premise in my larger argument for a return to the Medieval in composition (and a solution for the Church music crises).
Classical music is progressive. That's essential. But that's not the only thing that makes it classical. Something can be progressive and not classical. But something that isn't progressive is definitely not classical, no matter how hard it tries. Richard Wagner sounds very similar to John Williams sometimes. The difference is that Richard Wagner was from the 19th century and he leaned forward. John Williams is from the 20th century and he leans backwards. There is not enough in John Williams' music that is innovative in order for it, in a Classical setting, to be considered truly forward leaning. (As soundtrack music, I think he's extremely innovative and attention-arresting.)
There's a simple way to illustrate this - there are no composers who are considered universally classical that were not progressives and products of their time. An example of this would be, at random, Bach. It so happens Bach was extremely progressive. An example of this would not be a composer from the 20th century who was not progressive, because he would have to redefine what is historically the uniting principle of all Classical music in order to get his foot in the door.
But there are other things that make Classical music what it is. Classical music is an intentional expansion on the ideas of the Classical age. And by this, I do not mean Mozart, Salieri, and Haydn. I mean Aeschylus, Sophocles, and Euripides, who were asked by contemporaries the classical question, "What does it have to do with Dionysus?" The answer is, of course, that it has nothing to do with Dionysus. Classical is ars gratia artis - art for art's sake. It is secular. It is oriented around the stage, not Dionysus. It has nothing to do with the religious holidays anymore. It is now simply high drama for audiences in heels and ties. Or whatever they wore back then.
Classical music as we understand it is just this in a modern form. This can be shown clearly in the monodist movement of the 17th century. Monody was the birth of homophony, opera, and Classical music in its recognizable form. Their idea was not simply to nod back to Classical drama of ancient Greece but to manifest its rebirth and expand on its tradition. Out of this movement came a surge away from polyphony (music with many melodies) and towards the simpler homophony (music with one melody and some accompaniment) in high music.
This is why, properly speaking, anything before the rise of monodism or secular music in the West cannot be considered Classical music because it was not secular. It was religious. It answered the classical question posed above to Aeschylus differently - "It has everything to do with Dionysus." Or, more properly, Christ.
(Theodore M. Finney, A History of Music. Revised Edition. New York, 1947. As much as his analysis is atrociously anachronistic in some areas, he provides a helpful chart for those interested in the shape of music between the 9th and 18th century.)