The Teutonic Order continued to attack the region, claiming they were on Crusade (but also probably just looking to expand territory). Mindaugas' nephew (and later Duke) Treniota convinced Mindaugas in 1260 to ignore his Christian conversion and fight back against the Teutonic Knights. The attempt was ineffective, and Treniota allied with Daumantas, a duke of a northern province of Lithuania, to assassinate Mindaugas. (Daumantas is accused of the assassination of Mindaugas and two of his sons, but since Treniota succeeded Mindaugas as Grand Duke, it is pretty likely that he orchestrated the succession.)
Treniota was a staunch pagan and declared that Christianity was not officially part of Lithuanian culture. Unfortunately, a remaining son of Mindaugas, Vaišvilkas, killed him within a year and made himself Grand Duke. Vaišvilkas actually was baptized in a Greek Orthodox rite, and became so drawn into his new religion that he gave up the throne and founded a monastery, which he entered as a monk.
The ensuing decades include very few records, and we are uncertain of even the names of some of the Grand Dukes. Attacks by the Teutonic Knights continued, however, because Lithuania's rulers were not interested in Christianity.
Politically motivated marriages led to some Lithuanian rulers marrying Eastern Orthodox brides, but these made no lasting influence until Jogaila (c.1352 - 1434). Jogaila's mother wanted him to marry Sofia, the daughter of Prince Dimitri of Moscow. Jogaila would have to become Eastern Orthodox, and Lithuania would become a fief of the Grand Duchy of Moscow. A better option would shortly appear, however.
On 15 February 1386, Jogaila married Jadwiga (aka Hedwig), the "King" (Queen) of Poland. This created the opportunity for him to be crowned King of Poland as well as Grand Duke of Lithuania, a prospect that was so tempting that converting to his bride's Christian faith was no problem for him. He took the Christian name Władysław II Jagiełło. The baptism of his court and family followed, and Jogaila/Władysław sent to Pope Urban VI with a request to create an episcopal see at Vilnius, the capital. Vilnius got its cathedral in 1387.
Urban recognized Lithuania officially as a Roman Catholic state on 19 April 1389, although parts of Lithuania held out into the 15th century. (The illustration is called "The Baptism of Lithuania" by 19th century Polish painter Jan Matejko.)
So about Jadwiga: who was she, and was she King of Poland or Queen? That's a good story...for next time.