It could be done, I think, if you could get a single ruler to marshal the combined forces of the North. A Great Heathen Army, but writ even larger. They don't need to push the Franks or what have you back to the Rhine, you could make a defensible position out of Jylland. If they get beyond that, you've got the straits. Mix that with continued assaults on European coasts and Britain and eventually the Christians will move on. They gave up the Holy Land, after all, and it was easier to get there.
How was it easier to travel through the entire Mediterranean than just to hop across the Baltic?
Especially if you consider the insanity of the Crusaders.
I think that Kingdoms of Christendom would realize that fighting a force that looked forward to death in battle would be a losing endeavor after well a few hundred stubborn pointless battles.
You do realise that if you took part in a Crusade all your Sins were forgiven. Did they look forward to death? No not really, but they believed that if they died they would go straighter to heaven than any other Christian.
The Landscape of Scandinavia favors defenders.
Every countries landscape favours the defender. That's like a build in bonus for everyone. With certain measures it can be overcome. Technology, Logistics, Locals, etc.
Sure they could retreat into Scandinavia, away from the shore. But then they have to rely on hunting and provisions.
Not to mention Russian and Baltic areas would have people seeing Western Europe's Christian Adventurism as a threat and by extension give opportunity to the Vikings.
The Baltics? You mean those right next to Christian Poland and HRE? Ever heard about the Northern Crusades?
And Russian areas? So basically... Novgorodian Trade posts. Because that was everything there was back then.
The Kievan Rus' was as the name implies, more to the south and kinda Christian too. Moscow as a regional power did not exist.
Remember, the more desperate a force acts the more uneasy nearby nations become. It doesn't matter if it friend, foe, or neutral.
Uhm do you have any idea how far Novgorod or Kiev are from, let's say Skane? The people that would become the Russians had other problems than Central European conflicts. They weren't even unified or allied to each other.