I know this thread is very old, but I do know enough to answer it! Essentially, the Christianization of Lithuania was complicated as hell. It officially became a Catholic country in 1387. The King of Poland, Władysław II, was kin to the Duke of Lithuania, Vytautas. Cousins. Together, these two signed into reality the adoption of Christianity as Lithuania's official religion.

The Christianization of the population was quite arduous. The Yotvongians, a Baltic tribe in southwest Latvia and northwest Lithuania, were relatively quick to adopt due to acceptance by their leaders. For the rest of Lithuania, which, keep in mind, encompassed a much larger area than the smaller, modern-day Lithuania, leaders were less ecstatic about the introduction of Christianity. Essentially, the Romuvan religion had quite a zealous following. As Lithuania expanded, though, lots of pressure was placed onto the leaders. Much of their dominions were eastern Slavic ethnically, many of whom had converted during the conversion of Kievan Rus in the 10th century. Due to intermingling of cultures, many ethnic Lithuanians began using Christian names for their children, many even converting. In 1250, the Livonian Order officially adopted Christianity, and became hostile to Pagan Lithuania. Duke Mindaugas of Lithuania was attacked by the Livonian Order in 1249, in Nalšia. Mindaugas would actually stand down and have himself and his family baptized. Lithuania briefly became an official Christian Kingdom in 1253, however this only lasted until 1261. Mindaugas never ceased his practice of Paganism, and in 1261 evicted all Christians from Lithuania, effectively setting the conversion back to Step One. Even during "Christian" rule, most Lithuanians held onto their ancestral beliefs, and no major conversion effort was made by Western Christians.

Later Lithuanian dukes, most notably Gediminas, found it difficult balancing the influence of eastern Orthodox Christianity and Catholicism, further stagnating conversion efforts

Kęstutis, Duke of Lithuania in 1349, felt immense hostilities and pressure from the Livonian Order. He began negotiations with Pope Clement IV to convert the Lithuanian people. This plan also failed following a war between Lithuania and Poland over the Volhinya region, resulting in Kęstutis fleeing from Lithuania following a bad peace deal concerning Hungary. In this time, however, many Lithuanians had converted due to Slavic Orthodox and Catholic influence in Lithuania's extensive dominions. Mindaugas' successor, Vaišelga, would be baptized into Eastern Orthodoxy. Christianity would now be fairly accepted and practiced by the populous, but the state itself was still Pagan.

Finally, in 1387, the King of Poland and Duke of Lithuania officially christianized the country. This was following a Russian plan to further Russify Lithuania (the King of Poland was Russian-backed.) Conversion at this point was fairly smooth and uneventful.

SOURCES

Muldoon, James. Varieties of Religious Conversion in the Middle Ages. University Press of Florida, 1997

S.C. Rowell. Lithuania Ascending: A Pagan Empire Within East-central Europe, 1295-1345. Cambridge University Press, 1994.

Davies, Norman. Europe:A history. Oxford University Press

Daniel Z. Stone. The Polish-Lithuanian State, 1386-1795. University of Washington Press, 2001

Much of the info is also from my own memory.