Complex sentence: He again undertook the work of reform and held a council at Autun in 661 to denounce Manichaeism and was the first to adopt the Trinitarian Athanasian Creed.
Split into simpler sentences: He again undertook the work of reform and held a council at Autun in 661. The council denounced Manichaeism and was the first to adopt the Trinitarian Athanasian Creed.