The most authoritative source for the early Christianization of Rus' is an encyclical letter of Patriarch Photius, datable to early 867. Referencing the Siege of Constantinople of 860, Photius informs the Oriental patriarchs and bishops that, after the Bulgarians turned to Christ in 863, the Rus' followed suit. As was the case with the Bulgarians, the Patriarch found it prudent to send to the barbarians a bishop from Constantinople.