Explanation: Poland was long-Catholic, but Lithuania was one of the last pagan holdouts in Europe. For this reason, a crusade was undertaken by the Teutonic Knights (of horned helmets fame) to convert Lithuania… BY THE SWORD!
Poland, however, managed to convince Lithuania to convert mostly-peaceably instead, and formed the Polish-Lithuanian Commonwealth. This did not stop the Teutonic Order’s wars which involved mass plundering and total conquest of the Baltic. Curious! Wars, naturally, ensued with the Commonwealth, which the Teutonic Order eventually lost.
You’re oversimplifying a bit. Lithuania stayed pagan for a long time because of many different interest groups, both foreign and domestic.
Lithuania conquered a lot of Orthodox Rus lands and the Rus feared that they will be forced to become Catholic, for example. Also, there were few strong Catholic states in the region that could make strong allies.
Poland made Lithuania convert to Catholicism by accepting their grand duke Jogaila to become their king (by marrying their princess Jadwig), thus making him de jure leader of both Poland and Lithuania. For the Polish, it was an attempt to reduce Austrian influence on their state.
