Complex sentence: Puritanism has a historical importance over a period of a century (followed by 50 years of development in New England), and general views must contend with the way it changed character and emphasis almost decade by decade over that time.
Split into simpler sentences: Puritanism has a historical importance over a period of a century (followed by 50 years of development in New England). It changed character and emphasis almost decade by decade over that time.