The Cathedral of the Theotokos in Vilnius is the main Orthodox Christian church of Lithuania.

The cathedral was built during the reign of the Grand Duke Algirdas in 1346. It was constructed by Kievan architects with the blessing of Saint Alexius Metropolitan of Kiev and all Rus in 1348.

The Cathedral of the Theotokos is one of the most ancient churches of Vilnius, built before the christianization of Lithuania when the Grand Duchy of Lithuania was the last pagan state in Europe. It became an important spiritual centre for the growing Christian population of the duchy. In 1495 the marriage between Aleksandras of Lithuania and Yelena of Muscovy (Ivan III's daughter) was held in the cathedral in the presence of Saint Macarius. It was there that Yelena was buried in 1513.

After the conversion of Lithuania to Roman Catholicism, the Orthodox cathedral was protected by princes Konstanty Ostrogski and Konstanty Wasyl Ostrogski, who restored it after the collapse of the dome in 1506. After their deaths, the cathedral was taken over by the Greek-Catholic church in 1609 and was rebuilt in a style typical of the region.

Vilnius ([ˈvʲɪlʲnʲʊs] ( listen); see also other names) is the capital of Lithuania, and its largest city, with a population of 554,060 (838, 852 together with Vilnius County) as of 2011. It is the seat of the Vilnius city municipality and of the Vilnius district municipality. It is also the capital of Vilnius County. The first known written record of Vilnius as the Lithuanian capital is known from Gediminas' letters in 1323. Vilnius is classified as a Gamma global city according to GaWC studies.

The name of the city originated from the Vilnia River. The city has also been known by many derivate spellings in various languages throughout its history. The most notable non-Lithuanian names for the city include: Polish: Wilno, Belarusian: Вiльнюс, Вiльня, German: Wilna, Latvian: Viļņa, Russian: Вильнюс, Yiddish: ווילנע (Vilne). An older Russian name was Вильна / Вильно (Vilna/Vilno), although Вильнюс (Vilnius) is now used. The names Wilno, Wilna and Vilna have also been used in older English, German, French and Italian language publications. The name Vilna is still used in Finnish, Portuguese, Spanish, and Hebrew.

Saint Casimir Jagiellon (Polish: Kazimierz, Lithuanian: Kazimieras) (October 3, 1458 – March 4, 1484) was a royal prince of the Kingdom of Poland and of the Grand Duchy of Lithuania who became a patron saint of Lithuania, Poland, and the young.

A member of the Jagiellon dynasty, Casimir was born at Wawel, the royal palace in Kraków, and died at Hrodna. St. Casimir was the grandson of Jogaila and was the second son of king of Poland and Grand Duke of Lithuania Casimir IV and Queen Elisabeth Habsburg of Hungary. His maternal grandfather was the king Albert Habsburg of Hungary, king of Bohemia and "King of the Romans" of the Holy Roman Empire.

From the age of nine, St. Casimir was educated by Jan Długosz and Filippo Buonaccorsi (also known as Filip Callimachus). Casimir, eager to defend Christianity against the Turks, accepted the offer and went to Hungary to obtain the crown. His uncle Władysław III, king of Poland and Hungary, had been killed earlier at the Battle of Varna in 1444 while leading the Hungarian armies against the Ottomans. Casimir was unsuccessful in this undertaking and returned as a fugitive. His father, King Casimir IV, had him educated well concerning public affairs and when his brother Władysław, became king of Bohemia, Casimir became crown prince and heir-apparent to the throne of Poland.