Complex sentence: Irving's relationship to this community was, according to its members, somewhat similar to that of John the Baptist to the early Christian Church, i.e. he was the forerunner of the coming dispensation, not the founder of a new sect.
Split into simpler sentences: Irving's relationship to this community was, according to its members, somewhat similar to that of John the Baptist to the early Christian Church. He was the forerunner of the coming dispensation, not the founder of a new sect.