Pagan can mean, variously, pre-christian (eg: ancient Greek), or post-christian but unconverted (to any type of monotheistic religion), eg: Viking pagan, Baltic pagan, etc.
The last great pagan kingdom in Europe was that of Lithuania, which began its rise under Gedmyn in 1316 or so. Lithuania remained a pagan Kingdom until about 1386, when Jagiello, King of both Poland and Lithuania converted to Catholicism.
Pagan does not mean Heretic, as that refers to a follower of christianity who professes a doctrine different to that of the ruling church. Neither does it mean an Infidel such as a Moslem or a Jew.