The Rus Convert to Orthodox Christianity, 988 (legendarily)
According to legend, pagan Prince Vladimir of Kiev investigated Islam, Judaism, Latin-rite (later, "Catholic") Christianity, and Greek-rite (later, "Orthodox") Christianity before selecting Greek-rite Christianity for himself and his people in 988. There's some evidence that, instead, he converted as a condition for an alliance with the Eastern Roman Empire and also required his people to convert. In reality, Greek-rite missionaries had slowly been converting Slavic peoples for over a century, and Vladimir's conversion sped the process.
The difference between the Latin and Greek rites would result in the Great Schism of 1054, breaking Christianity into the separate and antagonistic Catholic Church and Orthodox Chuch. This would eventually result in the Catholic Teutonic Knights invading Rus lands in hopes of converting the Rus to Catholicism.