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. With some modifications, the story is repeated by Constantine VII in De Administrando Imperio, followed by several generations of Byzantine historians, including John Skylitzes and John Zonaras. That the imperial court and patriarchate regarded the 10th-century Rus' as Christians is evident from the fact that the bishopric of Rus' was enumerated in the lists of