Complex sentence: The pope, however, did not agree with the embassy, and wrote in reply to Coenwulf that the southern archbishopric must remain at Canterbury, and excommunicated Eadberht and authorized his expulsion from Kent if he persisted in keeping Æthelhard from Canterbury.
Split into simpler sentences: The pope, however, did not agree with the embassy. The papal reply to Coenwulf stated that the southern archbishopric must remain at Canterbury, as well as excommunicated Eadberht and authorized his expulsion from Kent if he persisted in keeping Æthelhard from Canterbury.