Alexander Scriabin was a Russian composer majorly influenced by Blavatsky and Theosophy.
Mystical ideas penetrated deep into pieces like Prometheus- Poem of Fire and The Poem of Ecstasy.
Great music. I've tried Prometheus stoned and it's overwhelming to say the least. He died before
he could realize his Mysterium. Mysterium was to be a multimedia event lasting several days at the base
of the Himilayas. He believed it would end the world and usher in a new age as all were dissolved into
pure bliss and ecstasy. A bit crackers, but genius. He composed circa the late Romantic era so it's a
possibility that he had experience with hash and/or opium as many artists were involved with them at the
time.

Another interesting composer is Olivier Messiaen of France who was deeply Catholic, but in a mystical
fashion after Sufi to Islam or Kaballah to Judaism.

As far as classical music is concerned those are the only two I can think of with mystical leanings.
I'm curious to know of any more.

J.S. Bach, Brahms and Vivaldy, to an extent. Swim's deepest spiritual experiences happened while listening to their music on strong doses of acid.

Invariably it was the opinion of swim's friends whom she introduced to acid.

In 60th there's been a lady scientist Helen Bonny who developed a program specifically for LSD trips. Once swim made an play list according to her recommendations. It is absolutely incredible. It is still swim's favorite.

Check the names on that list.

I looked into Bonny and her ideas regarding music programmin seem weighted in asthetic rather than objective concerns. Of, course asthetics do play a part in how people accept music, but psychedelics have a tendency to counteract social conditioning which is the substrate of asthetics. I think she just wanted to make a playlist of cool stuff to trip to the same way psychedelic users already do. She had some awesome picks, however.

I find this a difficult question, mainly because i don't know how you want to define mysticism.
However, i do know skriabin's music and if you like his music then there's a good chance that you will also like the music written by richard strauss (not to be confused with johan strauss), sergei rachmaninov, richard wagner and claude debussy.

I think that you will especially like strauss's alpine symfony, wich has realy deep sort of spiritual layers in it. It's a masterpiece, not only by it's technical briliance, but also because of the story it tells: it's story about a journey through the alps that starts at dawn and ends at night. In the meantime you're being led along dangerous cliffs and you'll reach the highest mountaintop from wich you can see the greater perspective.
But ofcourse this is all a metaphor for life itself, and you hear that throughout the whole piece. Especially the dawn 'scene' and the 'contemplation on the summit' scenes are just pure magic.

If you like skriabin, there is a good chance you'll be very gratefull that i've mentioned this piece.

I haven't heard the piece, but I do like what little I've heard of Strauss. I will try to find it though. Thus Spoke Zarathustra and Salome are great works.

I suppose it is a vague question considering the connotations of mysticism. Since I can't think of a way to narrow my question down I'll give up, but I am interested in the more progressive classical music, but I'm familiar with most of the big names (Debussy, Schoenberg, Ravel, etc.) I'm interested in any lesser knowns, however.

Hildegard von Bingen has some beautiful music inspired by her religious/mystical experiences. I particulary enjoy the piece entitled O Ignus Spiritus.

xx