Kievan Rus adopted Orthodoxy in like 700 AD.
That's just straight up false, i don't know what else to tell you.
Your argument that the totally-always-culturally-homogenous European nations not caring about slavs because they existed before and will exist after slavic nations is retarded considering there isn't a single European country that has existed even relatively unchanged since whatever bullshit point on the timeline you want to point to. The Treaty of Verdun around the same time as Kievan Rus was adopting Orthodoxy and beginning to flourish just drew an arbitrary line through Europe and said "ok this part is West Francia and this part is East Francia" which essentially created what we now consider "France" and "Germany" but those countries are just as arbitrary as "Poland" and "Serbia" when you really get down to it.
This is literally just a "Well AHKHTHUALLY nations are a recent phenomenon" argument, and it's a poor one. The reality is i can take any point in Germany or France and draw a direct line between the people 3000 years ago and the people now and when i'm talking about a peoples, i'm talking about culture, language, history, yes even religious practices, there's a reason the Holidays have different manifestations in different cultures and have different significance even though they might share a religion.
You can't do that with slavic nations, you can pick any point in Poland, any point in Serbian and we see the same thing. We have a peoples X, we follow them, follow, follow follow...we have a disconnect and all of a sudden we have this completely different people that do not share any qualities, rites, beliefs or even symbols with the previous ones. Admittedly things are much more complicated in the Mediterranean because in the Mediterranean and Europe to a lesser extent, you have constantly over lapping empires that either tried to fully eliminate or fully integrate whoever was there before them but my point stands.
So while you can cry all you want about how arbitrary this and that is, in reality you're no better than people that say how much of a social construct sex or gender or social roles are. It's literally the same argument, just the subject is different. The reason you aren't a hairless ape throwing your shit at other hairless apes now is because of that lineage, of that history, shared or otherwise.
The bullshit about "not having a proper language, religion or culture" is just invented out of whole cloth as well. Countries in that period are always composed of smaller cities, villages, regions that all speak their own dialect, have their own cultures, and often have differing religions (or different flavors of the same pagan/polytheist views). That's even true to this day, in modern countries like, for example, France, Germany, etc.
This is true...in the Neolithic, in the early bronze age, really most of the bronze age actually. Not when the slavs moved into europe in the slavic migration or slavic expansion, whatever the fuck you want to call it. The earliest, EARLIEST plausible date people give for the Slavic expansion is 6th century, most put at 7th. This is way passed the hunter gather phase, this is way passed the early settlers phase, this is passed the polis, passed whatever league you want to pick, you had very, very, very well established peoples in that period.
To condense my previous comment, the only reason slavs lived somewhere, is because other people put them there for their own interest.
EDIT:
I never made claims of cultural homogeneity or that European cultures are unchanged. Cultures change and evolve naturally, a static culture is a dead culture.
