Down in the Mother Church of Lithuania, in its crypt, you get a hint of where it originally got its Christianity from:http://en.wikipedia.org/wiki/Christianization_of_Lithuania
I just came across an extended scholarly treatment of this topic:
Lithuania ascending: a pagan empire within east-central Europe, 1295-1345 By S. C. Rowellhttp://books.google.com/books?id=i4hpVJ51y4oC&pg=PA149&dq=%22Until+1387+when+Lithuania%22&hl=en&ei=5nUMTpLAJKKesQK1qKH_CQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CC4Q6AEwAA#v=onepage&q=%22Until%201387%20when%20Lithuania%22&f=false
Wikipedia has a summary:
The Metropolitanate of Lithuania was a short-lived metropolitanate of the Orthodox Church in the 14th century. Created between 1315 and 1317, it had only two metropolitans and was discontinued in 1371. Its establishment was part of the Lithuanian entry into the rivalry for the religious control of the Rus' principalities between Galicia–Volhynia, the Principality of Tver, and the Grand Duchy of Moscow. The Byzantine Empire, seat of the Patriarch of Constantinople, generally preferred a united Metropolitanate of Kiev and all Rus' and was reluctant to divide its authority. Therefore whenever possible, the Byzantium would unite the metropolitanates. Facing opposition to actual physical division of the metropolitanates, the Lithuanians employed additional tactics: promotion of their own candidates to the seat of Metropolitan of Kiev and all Rus'. By the 1440s however, the final years of the Byzantine Empire, the Grand Duchy of Moscow had effectively won the dispute and became the new spiritual center of the Orthodox tradition in Eastern Europe.http://en.wikipedia.org/wiki/Metropolitanate_of_Lithuania
The Grand Duchy of Lithuania expanded east at the expense of Slavic Orthodox principalities of the former Kievan Rus'. While adhering to the pagan faith, Grand Dukes Vytenis and Gediminas understood the political importance of controlling the church. At the time Peter, the Metropolitan of Kiev and all Rus', supported by Galicia–Volhynia, rivaled with Mikhail Yaroslavich, Prince of Tver, who wanted to replace Peter with his own candidate. As a result of this dispute, the seat of the metropolitanate was moved to Moscow. Lithuania had a rather friendly relationship with Tver and perhaps the new metropolitanate was a way to support Mikhail Yaroslavich in his struggle with Metropolitan Peter, whose income was cut and authority in all of Rus' challenged. The Byzantine Empire, afraid of the growing influence of local dukes, generally promoted church unity within the Rus', hoping that a strong united patriarch would be able to resist political intrigues. Therefore it is unclear why it agreed to establish a new metropolitanate; later the Byzantine authorities regarded it as an "anomaly" or the "result of confusion." Possibly, Emperor Andronikos II Palaiologos, involved in wars with the Ottoman Empire over Asia Minor, needed military and financial assistance, both of which Lithuania could provide. The emperor established the metropolitanate while Patriarch John XIII ordained the prelate.
Metropolitan Theophilus (ca. 1317–1330)
The Metropolitanate of Lithuania, with episcopal see in Navahrudak, had two suffragan bishops in Turov and Polatsk. From 1317 to 1330 it seems that there was only one metropolitan bishop, Theophilus, of Rus' origin. A surviving list of his property shows that Theophilus traveled extensively around the Rus' principalities and presented expensive gifts to prominent rulers of the region, perhaps as part of a campaign to become the Metropolitan of Kiev. After Peter's death in 1326 however, Theophilus and a candidate presented by Moscow were rejected by Constantinople as too political. Instead, they appointed independent Theognostus as the new Metropolitan of Kiev and all Rus'. When Theophilus died in 1330, Theognostus succeeded in restoring unity in the Rus': claiming that there were too few Christians in pagan Lithuania, the seat of the Metropolitanate of Lithuania was left vacant. In case a need would arise in future, a new metropolitan could be appointed. In the meanwhile Theognostus would have authority over all Rus' and Lithuania.
Metropolitan Roman (1355–1362)
After Theognostus' death in 1353, at first Grand Duke of Lithuania Algirdas did not attempt to revive the Metropolitanate of Lithuania, but promoted his own candidate Teodoryt to the see of Kiev and all Rus'. When he failed to gain support in Byzantium, Teodoryt turned to the schismatic Bulgarian Orthodox Church and received ordination there. Such actions may indicate that Algirdas envisioned an autocephalous church of his own. The angered Byzantines forced Algirdas to change his tactics. He now supported Roman, a monk from Tver and relative of Algirdas' wife Uliana, and even promised to convert to Orthodoxy in exchange for ordination of Roman. Patriarch Callistus I, rivaled by Philotheus Kokkinos, agreed and ordained Roman as Metropolitan of Lithuania in 1355.
Roman attempted to assert his authority over all Slavic lands of the Grand Duchy, even though they belonged to the Metropolitanate of Kiev and all Rus', whose patriarch Alexius resided in Moscow. In 1356, after diplomatic struggles, Callistus I united the Metropolitanates of Lithuania and Galicia under Roman, while Alexius retained his title. The rivalry continued however until Roman's death in 1362, when Lithuania–Galicia were placed under control of Alexius. In 1371, the Metropolitanate of Lithuania was officially lowered to the rank of bishopric and placed under jurisdiction of the Metropolitan of Kiev and all Rus'. However, it did not end the political rivalry for religious influence in Rus'. Algirdas successfully promoted his candidate Cyprian while Alexius was still alive. In 1415, Grand Duke Vytautas attempted to re-establish the Metropolitanate of Lithuania and promoted Gregory Tsamblak. The rivalry effectively ended in 1448 when Moscow began electing the patriarchs independently without approval from the Byzantium, which collapsed in 1453.
More is here:
Byzantium and the Rise of Russia: A Study of Byzantino-Russian Relations in the Forteenth Century By John Meyendorffhttp://books.google.com/books?id=bFRXmG5GdkEC&pg=PA188&dq=Byzantium+Meyendorff+Lithuania&hl=en&ei=koUMToiuD_TJsQKs9riuCg&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDIQ6AEwAQ#v=onepage&q=Byzantium%20Meyendorff%20Lithuania&f=false
The Metropolitanate would have had jurisdiction here:
or more detail
Here is the Statute of the Grand Duchy of Lithuania, written in its official language, Ruthenian in Cyrillic script