Complex sentence: Sumner did not make known, at least publicly, his reasons for leaving the ministry, but he and historians suggest that it might have been a loss of belief and/or a dim view of the church and its clergy.
Split into simpler sentences: Sumner did not make known, at least publicly, his reasons for leaving the ministry. However, he and historians suggest that it might have been a loss of belief and/or a dim view of the church and its clergy.