Lithuania's oldest Church is the catholic St. Nicolas Church (Sv. Mikalojaus Baznycia) which was founded by German merchants in 1320.
The red brick Gothic building was the only church in Vilnius where Lithuanian's were allowed to go to church services during the Polish occupation (1920 - 1939).
The little St. Nicolas Church can be found in the western part of Vilnius Old Town.
Address: St. Nicolas Church, Mikalojaus gatve 4, Vilnius
If you want to see really old, little church without glitter of baroque, go to look at Gothic church. Supposedly, it is oldest in Vilnius and mostly remained from about year of 1320. In inner yard is the sculpture of St Christopher, patron of Vilnius city.
I noticed there are many churches here and this one was the first I saw, in the centre of the old city.