I think its best to choose music which is sung in English.
I love "Jesu Joy of Man's desiring". "Ave Maria" by Schubert is beautiful but it is very RC. Panis Angelius is lovely. Would you want your year 6 class to hear music performed by children of their age.
"Pie Jesu" and modern classical piece and is a requim mass.
There are quite a few "Passions" - at least a couple of famous ones by Bach. They tend to get performed in big churches and cathedrals in Holy Week, so are still used for "teaching". Handel's Messiah, that's another biggy with lots of scripture in it.
At the other end of the scale, how about more modern church music that isn't actually hymns. eg stuff by Roger Jones
Hi. For my y6 lesson on Wednesday I need some examples of how Christians teach about Christianity through music. Obviously I can find some hymns, but I also wanted some amazing pieces of classical music written with a Christian message .... just can't really think of anything right now!