In 634 Pope Honorius I sent a bishop called Birinus to convert the Saxons of the Thames Valley to Christianity. King Cynegils of Wessex gave Dorchester to Birinus as the seat of a new Diocese of Dorchester under a Bishop of Dorchester; the diocese was extremely large, and covered most of Wessex and Mercia. The settled nature of the bishopric made Dorchester in a sense the "de facto" capital of Wessex, which was later to become the dominant kingdom in England; eventually Winchester displaced it, with the bishopric being transferred there in 660. Briefly in the late 670s Dorchester was once more a bishop's seat under Mercian control.