b) It believed that changing any part of the church was heretical--against Christianity.
The Eastern church rejected the changes in religious practices that the Western church made because it believed that changing any part of the church was heretical. The Eastern church, also known as the Byzantine church, had its own religious traditions and practices that had developed over time and they saw any deviation from those traditions as a form of heresy. They rejected the changes such as the use of unleavened bread in the eucharist and the use of the filioque in the Nicene creed, as they saw it as a deviation from the original teachings of the church. This led to a separation between the Eastern and Western church, known as the Great Schism, in 1054 AD.