Complex sentence: The Chalcedonian Creed was adopted at the Council of Chalcedon in 451, which is one of the seven Ecumenical councils accepted by Eastern Orthodox, Catholic, and many Protestant Christian churches.
Split into simpler sentences: The Chalcedonian Creed was adopted at the Council of Chalcedon in 451. That Council is one of the seven ecumenical councils accepted by Eastern Orthodox, Catholic, and many Protestant Christian churches.