Complex sentence: He had been posted to Arabia as chaplain for all non-Anglican and non-Roman Catholic denominations (including by default atheists and agnostics), where his preprepared topical sermons went down like lead balloons among the young airmen.
Split into simpler sentences: He had been posted to Arabia as chaplain for all servicemen who did not designate themselves as Anglican or Roman Catholic. His preprepared topical sermons went down like lead balloons among the young airmen.