Complex sentence: As a religious sect, reportedly, the Samaritans broke away from the mainstream Jews around the fifth century BCE, but according to Samaritan accounts it was the southern tribes that left the original worship set forth by Joshua.
Split into simpler sentences: As a religious sect, reportedly the Samaritans broke away from the mainstream Jews around the fifth century BCE. According to Samaritan accounts, it was the other, southern tribes who left the original worship as set forth by Joshua.