Urlicht is indeed extraordinarily beautiful. But it can only fade away, and only five years later, Mahler acknowledged that even Paradise is tainted;
Johannes das Lämmlein auslasset,
Der Metzger Herodes d'rauf passet.
Wir führen ein geduldig's,
Unschuldig's, geduldig's,
Ein liebliches Lämmlein zu Tod.
Sankt Lucas den Ochsen tät schlachten
Ohn' einig's Bedenken und Achten.
I think it is this realisation that underpins most of his subsequent composition, particularly the progression through the angst and despair in the fifth, sixth and seventh symphonies, and Das Lied von der Erde (Der Abschied is arguably the most spiritual piece that Mahler wrote). It would be possible to make a similar argument for Wagner or Bruckner, or other 19th century composers, as, for example, Kierkegaards existentialism and Darwins theories began to exert increasing influence and question the nature of existence and meaning.
Theres a strong case for any of Beethovens late string quartets to be the most spiritual works written. But Id suggest a more humble work the Earth and Air and Rain song cycle by Gerald Finzi, based on some of Thomas Hardys late poetry for consideration. Not every song in this cycle is successful, but Waiting Both, The Phantom, The Clock of the Years, In a Churchyard, and Proud Songsters bring together poetry and music in a way I find deeply moving; in performance, it can create a sudden realisation and rapt attention from the audience. The last song addresses the same questions that Mahler was grappling with;
These are brand-new birds of twelve-months' growing,
Which a year ago, or less than twain,
No finches were, nor nightingales,
Nor thrushes,
But only particles of grain,
And earth, and air, and rain
