I can give you the current perspective from the Polish point of view.
The animosity between Poles and Russian doesn't stem from religion. However, adopting different religions 1000 years ago put Poland and Russia in different "spheres of influence" and gave birth to numerous cultural differences. Nobody has doubts that Russia is Eastern Europe. So why do Polish people hate it when you call them like that? Because we've always considered ourselves part of the Western world, as opposed to the "wild East" - Russians, Mongols, Tartars, etc. In this way, Poles kind of looked down on Russians, Ukrainians, and other Eastern European nations, as someone less civilized. I'm using past tense on purpose here, I wouldn't say it's an issue anymore.
And, much more important, Poles look at Russia through "imperialist" glasses. Because of more than a century (1795-1918) when a large part of Poland was under Russian control, and 45 years of communism, when all of Poland was under Russian control, Poles are extremely suspicious of all signs of Russian imperialism. Take, for instance, the story of Nord Stream - the pipeline from Russia to Germany. In Poland it was called new Ribbentrop-Molotov pact, betrayal, etc. and it strengthened anti-Russian sentiment like nothing in the last 20 years. Also, Putin's declarations that if the American anti-missile system is placed in Poland, Russia would re-target their nukes at Poland - it was seen as outward hostile, although about 80% of people were actually against having the missile shield. Both things strengthened the stereotype of Russia as an imperialist country. The problem is that unfortunately Russia doesn't seem to treat Poland as a partner (we won't talk to you because you're now EU so we'll talk with Brussels, because the EU is an equal partner with Russia). This is not helping in any way. Also, Russians don't understand that Poles didn't treat the end of WW2 as "liberation" but rather as a change of occupation (in my opinion only partly true). This causes issues when we start discussing history.
As for the issue with pre-war Polish eastern territories (as someone mentions here), I think there is no problem with it - nobody serious makes any claims about them now.
I believe the current younger generation isn't particularly anti-Russian, but without some serious discussion there is no way we can put aside the differences, we just don't understand the other side. Also, more contact would be most helpful (we should have visa-free travel!!!). Imagine an average Polish person who has never been to Russia and hasn't met any Russians in person, and gets all this media pulp about Russia...
Fortunately, this grudge is not so much against Russians as people, it's against their rulers. So, we seem to understand each other as long as we don't discuss WW2 or Putin :) We're Slavs, anyway, though very different Slavs.
Greetings to all Russians, I hope we can bridge the gap ;)