Complex sentence: Birinus's original commission entailed preaching to parts of Britain where no missionary efforts had reached, and may have included instructions to reach the Mercians, but in the end Birinus stayed in the West Saxon kingdom, or Wessex as it became known.
Split into simpler sentences: Birinus's original commission entailed preaching to parts of Britain where no missionary efforts had reached, and may have included instructions to reach the Mercians. But he ultimately remained in the kingdom of West Saxon kingdom, or Wessex as it became known.