Your 3 Favorites from a Past Great Composer ...

Because there are universal themes within religions that aren't uniquely religious in nature. To use Evangelion as an example, its use of the cross does symbolize sacrifice, death and rebirth; but it removes the "Jesus, son of God, died for our sins" aspect. Parsifal is pretty much the same. Schopenhauer and Wagner were both atheists, but they both adopted a Buddhist-like philosophy on life, where human will lead to suffering and the way out of the cycle of suffering was through the negation of the will, which is why Schopenhauer came to admire the asceticism of monks. He also thought art was more effective than philosophy or logic in this negation because it could provoke meditation on the nature of life, which in itself is a type of negation.

Religions often naturally foster a similar perspective in which earthly will is negated by meditating on the universal and eternal, even if the nature of the "universal and eternal" differs among religions. One way they do this IS through art, through stories, parables, allegories and symbols that reinforce those ideas. Take away the "son of God" aspect and the story of Jesus is basically about a man who negates his own will and teaches others to do the same before he's killed by those of the world being negated, and by those who were supposed to be the representatives of the life/philosophy he was teaching and living, but had really just become corrupted by the will/world themselves.

Parsifal is very much about that world where religious symbols have become atrophied and the representatives of that philosophy have been corrupted by the world. Parsifal is the redeemer precisely because he ventures out into the world and becomes empathetic with the suffering it entails. But Parsifal isn't a god-figure redeeming man through his sacrifice; he's more representative of the artist who redeems the value and power of the symbols (and rituals) that were supposed to remind us of these truths and provoke that same empathy. He's very much the "Middle Way" between the sensually indulgent will to life and the ascetically abstract negation of that will. These themes are pretty much encoded into both the music and dramaturgy.

I've read that Guardian piece before, but it's nothing more than a shallow op-ed. The author may have been confused by Parsifal (many were, and still are), but Parsifal is not a "a confusion of Christian and Buddhist themes." The only confusion is in the fact that people can't seem to see past the religious symbolism to the non-religious themes underneath, which is perhaps appropriate since that failure seems to be one of the opera's themes.