This is an invention of the show. I can't speak definitively about the entire course of the Middle Ages, but the idea is suspect to me. I cast my net wide and did a quick search, and the only thing I came up with was some anti-Catholic propaganda regarding the Inquisition—surely not a reliable source.

In fact, the whole notion of 'apostasy' seems somewhat out of place for the Viking Age. People did lots of stuff that bishops condemned, but they weren't executed for it. Of course, if a community tried to shuffle off church rule and with it the administrative arm of the king, then these actions could be cast as an uprising or revolt, and no holds were barred.

Thus Charlemagne advanced his control over conquered Saxon territories in part by establishing church institutions. When the Saxons revolted against him and vented their discontent on the religious communities in their midst, Charlemagne massacred some Saxons and deported others to be resettled elsewhere. Certainly not a great fate, but it was also not the crucifixion of individuals for apostasy.