ENThe article concerns historical Lithuania, i.e. the Grand Duchy of Lithuania, which was established in the second half of the 13th and the first half of the 14th century, reaching its apogee during the reign of Grand Duke Vytautas (1392-1430), who officially presented himself as magnus dux. Władysław Jagiełło (d. 1434), who succeeded his deceased father Olgierd from 1377, used the title dux supremus after ascending the Polish throne in 1386 and transferring direct power to designate his hereditary country. They both ruled in close cooperation with each other. However, during their reign, the principles of the union were changed several times, from the incorporation of the Grand Crown Duchy (Krevo - 1385) to permanent unification (Horodlo - 1413). The final integration, combined with internal political dualism, took place in 1569 (Union of Lublin) in the face of the prospect of the extinction of the Jagiellon dynasty, which finally occurred three years later with the death of Sigismund Augustus. The Grand Duchy of Lithuania then occupied approximately 90% of the territory of Rus', inhabited by the Orthodox population, whose Christianization in the Western order began at the end of the 10th century. However, significant traces of paganism survived in the Middle Ages, which explains why scholars have used the term "double faith" (dvojevierije) to refer to this period. In 1596 (the union concluded in Brest-Litovsk on the initiative of the Holy See) the eastern religion was subordinated to Catholicism, which caused strong resistance and divided the faithful of the "Ruthenian religion", the followers of the old Orthodox religion (disuniats) and those dependent on Rome (uniats). At that time (from 1569), Podlasie, as part of the Polish-Lithuanian Commonwealth, separated from the Grand Duchy of Lithuania and, like Volhynia, Kiev and Podolia in Ukraine, then became part of the Crown.Although Lithuania still possessed Belarusian lands, the proportions between the two nations changed (1:2). The latter were already well integrated in the cultural sphere, although from the 15th century Russianism gradually replaced the Polonization of the magnates and wealthy nobility. This was especially visible in the religious sphere during the Counter-Reformation, i.e. during the Catholic Reformation. The main topic of our dissertation is the process of historical Christianization of Catholic Lithuania, which can be divided into the following stages: 1) an unsuccessful attempt to renounce paganism in the mid-13th century, 2) the baptism of Upper Lithuania (Aukśtaitija) in 1387 and Samogitia in 1413, and its consequences, 3) the storm initiated by the Protestant Reformation in the 16th century, 4) the counter-offensive of the Church and the revival of Catholicism leading to the fall of the Polish nobility, 5) the post-partition period after 1795, 6) the formation of the modern Lithuanian nation in the last quarter of the 19th century, 7 ) the formation of the modern Lithuanian nation in the last quarter of the 19th century and the rebirth of the state after World War I. Religion became the subject of ethnic conflicts. After both countries concluded concordats with the Holy See in 1925, the old structures of church organization, dating back to the times of the nobility, were abolished.