Arthurian legend evolved over a period ranging from the end of the pagan era to the High Middle Ages, when the Catholic Church was the most powerful institution in Europe. The legend therefore includes a mix of pagan elements, such as Merlin and the Lady in the Lake, and Christian elements, such as the quest for the Holy Grail. Cornwell locates The Winter King in a part of history where Britain is in a constant push and pull between its pagan traditions and emergent Christianity. The conflict is in part generational. At the outset, High King Uther is a vestige of the old ways, aware of Christianity but finding it to be more of a personal annoyance. Arthur was a ward of Merlin, but he is much more circumspect in his own views on religion, as he recognizes the growing appeal of Christianity and, most importantly for his purposes, the burgeoning financial power of the Church. Therefore, by the start of the novel, pagans generally enjoy political and military power while Christians are winning hearts and minds, which translates into donations.

In this roughly equal but asymmetric balance of power, there are fanatical elements within each camp trying to destroy the other.