Appointed by Oswiu as a subordinate ruler, Alhfrith apparently clashed with his father over religious policy, which came to a head at the Synod of Whitby in 664.
His mother was Oswiu's first wife, Rieinmelth, granddaughter of king Rhun of Rheged; the marriage also produced a daughter, Alhflaed.
He evidently removed Œthelwald and installed Alhfrith as under-king (a largely autonomous ruler) owing homage to him.
[5] Alhfrith initially followed his father in adhering to Celtic Christianity, which practised certain customs at odds with those endorsed by the Bishop of Canterbury and Continental Europe.
Around 658, Alhfrith's ally Cenwalh introduced him to Wilfrid, a Northumbrian churchman who had studied in Europe and strongly advocated the Roman customs.
[7] In 664, Oswiu convened the Synod of Whitby to determine which form of Christianity Northumbria would follow; Alhfrith served as a key proponent of Wilfrid and the Roman system, which ultimately prevailed.