As Northern/Eastern Europe converted from Paganism to Christianity in the 11th century, how was the conversion enforced and what were the consequences for remaining with the old faith?

I can only really speak to Scandinavia at this time.

At the time of the 11th century, Christianity had been in Scandinavia for decades but seems to have been largely relegated to the more urbanized coastal areas with the more inland areas remaining heathen for as much as century in many parts of the peninsula.

As to how it was enforced, its a bit of a tricky issue. Many areas took to Christianity pretty readily. Some monarchs, infamously St. Olaf (though his exact role is under increasing scrutiny) were rather brutal in repressing heathen practices. Stories, and I must emphasize that they are stories, talk of him tearing down heathen shrines, executing heathens who refuse to convert, and similar brutal practices. However it is not possible to reconstruct a broader practice of repression across the whole area. It depended quite a bit on the character of the king and his own zeal in enforcing the conversion. Rural and less developed areas are infamously stubborn in religious practices and they would not have been completely converted for decades, despite significant pressure coming from the coast from Christian missionaries.

However some common elements can be identified in the more well developed areas. The upper classes, namely the royal and noble families, converted first and it spread pretty quickly among them. There are some exceptions of course, but by the 11th century most of the nobility of Scandinavia was Christian, at least nominally, how long it took for the theology to sink in is a much more complex issue.

As for penalties for not being Christian, it depended greatly. In Iceland there was a short period of religious tolerance between the two, but eventually Christianity was established as the sole religion, but by that point it seems to have been the majority religion anyways.

There might be evidence of trials or something similar for those who remained heathen, but I have not seen or read any of them.

Sources: Abram, Christopher. “The Two Modes of Religiosity in Conversion-Era Scandinavia,”

and the collection that article is in, Conversion and Identity in the Viking Age, ldar Garipzanov