 The most authoritative source on the first Christianization of the Rus' is an encyclical letter of Patriarch Photius of Constantinople, datable to early 867.Referencing the Rus'-Byzantine War of 860, Photius informs the Oriental patriarchs and bishops that, after the Bulgars turned to Christ in 863, the Rus' followed suit so zealously that he found it prudent to send to their land a bishop.Byzantine historians, starting with the continuation of Theophanes the Confessor, assumed that the Rus' raid against Constantinople in 860 was a Byzantine success and attributed the presumed victory to the Intercession of the Theotokos.