A Norse Wank. 878, Guthrum defeats Alfred the alt-Great. Dead Alfred is no longer building monasteries to convert the heathen. English cultural practices, use of runic writing in daily life, dress, intermarriage is all tending Norse, like it was in the Danelaw OTL until the reconquest reversed the trend.
Something, a revolt, a smelly priest, whatever, cause the Norse ruling class to turn decisively against Christianity, killing the Priests in the next 20-30 years. Jesus is increasingly seen as the God of the conquered rather than Conquer as, at the same time, the Norse-Saxons finish the conquest of Ireland, Wales, and Lowland Scotland and the process repeated.
The Norse now have a model of cultural assimilation and transmission and uncontested rule, as well as larger population to draw on, with the Norse-Saxons being part of this growing Pagan world. Writing on Vellum in a Runic-cursive becomes common.
This cultural package, somewhat garbeled and adapted, is transmitted to the trade routes to proto-Russia and the Baltic countries and to Iceland and to a Vinland reinforced by Norse Saxons, which leads to a successful, but more gradual conquest of the Americas. But perhaps little things, like Maize and bigger turkey eggs increasing the nutrition of the Norse but entering the rest of Europe more slowly creates an edge.
The end result is the Baltic, North Sea and Channel being as much a dividing line as culturally as the Mediterranean, a separate Civilization anchored in England and Eastern Europe, with the Christian world petering out somewhere around the Vistula
Europe circa 1400 is smaller, more defensively minded than otl.