Complex sentence: The Bulgarian annexation of Serbia in 924 was important for the future direction of the Serbian church, and by then, at latest, Serbia must have received the Cyrillic alphabet and Slavic religious text, already familiar but perhaps not yet preferred to Greek.
Split into simpler sentences: The Bulgarian annexation of Serbia in 924 was important for the future direction of the Serbian church. By then, at latest, Serbia must have received the Cyrillic alphabet and Slavic religious text, already familiar but perhaps not yet preferred to Greek.