It is in Cynegils' reign that the first event in West Saxon history that can be dated with reasonable certainty occurs: the baptism of Cynegils by Birinus, which happened at the end of the 630s, perhaps in 640. Birinus was then established as bishop of the West Saxons, with his seat at Dorchester-on-Thames. This was the first conversion to Christianity by a West Saxon king, but it was not accompanied by the immediate conversion of all the West Saxons: Cynegils' successor (and probably his son), Cenwealh, who came to the throne in about 642, was a pagan at his accession. However, he too was baptised only a few years later and Wessex became firmly established as a Christian kingdom. Cynegils's godfather was King Oswald of Northumbria and his conversion may have been connected with an alliance against King Penda of Mercia, who had previously attacked Wessex.