Original: A schism within the church in 1054 A.D. aggravated earlier divisions that emerged at the  451 Council of Chalcedon and was followed  by the Crusades from the west to rescue the east from Muslim conquests that had begun to encroach on the Byzantines.
Simplified: A schism within the church's authority in 1054 was added to the earlier division that had persisted since 451 and was followed by Crusades from west to rescue the east from Muslim invasion.