Complex sentence: Rædwald's marriage to a member of the royal dynasty of Essex helped form a diplomatic alliance between the neighbouring kingdoms of East Anglia and Essex and his conversion in Kent would have affiliated him with Æthelberht, so bringing him directly into the sphere of Kent.
Split into simpler sentences: Rædwald's marriage to a member of the royal dynasty of Essex helped form a diplomatic alliance between the neighbouring kingdoms of East Anglia and Essex. His conversion in Kent would have affiliated him with Æthelberht, so bringing him directly into the sphere of Kent.