The Sasanian Empire turned Caucasian Albania into a vassal state in 252, while King Urnayr officially adopted Christianity as the state religion in the 4th century. Despite Sassanid rule, Albania remained an entity in the region until the 9th century, while fully subordinate to Sassanid Iran, and retained its monarchy. Despite being one of the chief vassals of the Sasanian emperor, the Albanian king had only a semblance of authority, and the Sasanian marzban (military governor) held most civil, religious, and military authority.