You need to read something like the Great Bruce Quarrie's books on the SS which have chapters on the sort of folk who joined.

It is a bit of an odd point though: technically the majority of the British population were nominally Christians but that does not mean the major political groups were all 'Christian' parties or that there was any religious angle involved in the politics of the War. Vast numbers of the British and Commonwealth Armies were certainly not Christian but it was hardly a relevant point when the wolf was at the door.