Inspired by the "Where do you place Hungary" -thread I decided to resurrect this one, as I did some research. Like Hungary, Finland has been between the West and the East, but how much really? I think at least Ruth4Truth will find this interesting. I made some maps to demonstrate this.
The first map shows the major peace treaties between Sweden and Novgorod, later Russia. And more.
I drew these borders on an map showing medieval stone churches existing today. As you can see most of them are in SW Finland, which explains why Finnish culture up to the mid 19th century was dominated by SW culture well integrated to the Swedish Crown, while Eastern Finland where West and East really met were just a sideshow. The brown area show the area of permanent agricultural settlement around 1500, and as you can see Northern and Eastern Finland was very sparsely populated. The so called "pizza triangle" Helsinki-Turku-Tampere is still the area where over half of the Finns live in, just like it did in 1500.
The Treaty of Nöteborg in 1323 between Sweden and Novgorod defined the the borders of Finland for the first time. This split the populace into Catholic Europe and Orthodox Europe. Part of the Karelian Isthmus and especially the newly founded city of Viipuri fell under Western influence, with the nobility and bourgeoisie quickly becoming Swedish-German-Danish.
The next new movement came with the Treaty of Teusina in 1595. In Savonia, historically being split by the 1323 treaty, experienced a huge population boom in the late 1500's, and was to colonise the Central Finnish lakeland and by a royal decree even encouraged to do so by getting tax exemptions. Thus, the Orthodox Eastern Karelians were pushed eastwards.
After the treaty of Stolbovo in 1617 which made Sweden a Great Power, defined most of the historical borders of Finland. Now when administration and centralisation of power had developed and Savonia was being overcrowded, the Swedish kings pursued an aggressive settlement policy. Savonians were to move to the conquered territories and were followed by an almost inquisitor-style clergy forcefully converting Karelians, Izhorians, Veps and Votics to Protestantism. Most Karelians refused and fled either to the Tver region of Russia or Olonets Karelia. The Eastern Orthodox Karelians started to call the Protestant Finns "Ruotsiksi" = Swedes.
This dialect map shows where most ethnic Karelians resettled. The northern population settled in areas 2 and 3, while the people on the isthmus and Ingria fled to the Tver region.
The settlement policy was extremely fast and effective. In Ingria (the Saint Petersburg region) in 1695 74% of the inhabitants were Finns, 5% Swedes and maybe 2% Germans. Thus Ingria had lost all of its so-called "Eastern influence". When Russia gained power and conquered Ingria back, almost all Ingrian Finns fled back to Östlandet (The Eastern Country = Finland), and ethnic Russians took over. By this time almost all connection between Finnic-speaking Orthodox people and Finns living under Swedish rule was lost forever, if you don't count people living on the fringe areas of Finland.
The next map is from the 1840 census. The blue border here is from the 1323 treaty, which show why the southwestern Finnish culture became dominating defining us. Even using a very conservative definition marking the division between Western and Eastern Finnish dialects (red), show that the majority of the population was always on the Western side. It wasn't even before the 1850's when scholars started to realise that we have a very rich component who really are between West and East. Elias Lönnrot, the creator of the national epic Kalevala went to Eastern Finland and to Russian Karelia to gather knowledge, as Western Finland was so "latinised". This layout still exist today, the culture of Finland is around 90% dominated by the Southwestern Helsinki-Turku-Tampere pizza slice, and the rest are only provinces filled with people having a funny accent.
So as a conclution I could say that while Finland was originally genuinely a territory where West and East interacted and succeeded in it, the power of the Swedish crown was too much. By 1700 99% of Finns were Protestant and servants of the Swedish crown, and we lost the Eastern connection forever, largely by colonising areas that weren't ours, and driving the eastern component out. Where Karelians and Finns really could meet and converse were in the most sparsely populated regions, merely an forgotten outback.