In 1278, Novgorod gained control over Eastern Karelia, a region inhabited by people who spoke Eastern Finnish dialects. Meanwhile, Sweden took control of Western Karelia during the Third Swedish Crusade in 1293. From this point, Western Karelians became part of the western cultural sphere under Swedish rule, while Eastern Karelians were influenced by Russia and Eastern Orthodoxy.
Despite maintaining linguistic and ethnic ties to the Finns, Eastern Karelians became culturally distinct, aligning more with Russian traditions and Orthodox Christianity. This division was solidified by the Treaty of Nöteborg in 1323, which set the eastern border of what would later become Finland, marking the boundary between Catholic and Orthodox Christendom in the north. This historical separation left a lasting cultural and religious divide between Western and Eastern Karelia.