Answer
Władysław II Jagiełło, the Grand Duke of Lithuania, played a pivotal role in spreading Christianity in Lithuania. His marriage to Jadwiga, Queen of Poland, in 1386, marked a significant turning point. As part of the agreement, Jagiełło converted to Catholicism and agreed to Christianize Lithuania. Through his leadership and influence, Lithuania officially adopted Catholicism in 1387, marking a crucial moment in the religious and cultural transformation of the region.