Good concept, but could use some improvement. :)
This is not a bad piece, not a bad piece at all, but here are some suggestions to make it better.
1) It seemed very cluttered at times. Try slowly adding on new melodic lines, and ideas, and introduce the listener to each idea. That way, it will not sound very cluttered from the start.
2) Though the string sample sounds very good, there is no reverb to make the choir or the string seem live. Add some reverb, or more reverb, and work on things like dynamics. Don't count on more melodic lines to push the piece forward dynamically, because it will just end up in disaster.
3) You need a defined ending. The end just seems to fizzle away like all that important polyphony and counter-point was useless. The last impression the song will make on your listener is the ending, so try to make it as grand as your overall piece.
Good job! :D For a song in minor, and your first classical submission, not bad work at all. I could really see this as a piece on piano, too. :)
I can't wait to hear more! ^_^
~Smartpoetic