Pagans. Not any kind of Christian, so not heretics. Lithuania was the last pagan country in Europe, until Grand Duke Jogaila was bribed to convert in 1386, with the offer of Queen Jadwiga of Poland & her kingdom. The Prussians & Latvians worshipped the same gods as the Lithuanians until they were converted by force, but the Lithuanians were too strong militarily for that.
You may be confused by the competition between the Orthodox & Catholic churches to convert the Lithuanians, & the view at the time of the conversion of Jogaila as a specifically Catholic victory, a score against the Orthodox, rather than a more general Christian one.
