Complex sentence: He has written widely on religious matters, from Celtic Christianity to the theology of sacrifice and the future of spirituality in Britain in addition to publishing on the subjects of hymnody, monarchy, the Victorian era, Gilbert and Sullivan and musical theatre.
Split into simpler sentences: He has written widely on religious matters, from Celtic Christianity to the theology of sacrifice and the future of spirituality in Britain. In addition, he has published works on the subjects of hymnody, monarchy, the Victorian era, Gilbert and Sullivan and musical theatre.