Complex sentence: Troeltsch sought to explain the decline of religion in the modern era through a description of the historical evolution of the role of religion in society, describing European civilisation as having three periods: ancient, medieval and modern.
Split into simpler sentences: Troeltsch sought to explain the decline of religion in the modern era through a description of the historical evolution of the role of religion in society. He described European civilisation as having three periods: ancient, medieval and modern.