Arthur wants to strengthen his alliances among the British kings with the hope of uniting them against the invading Saxons, but Merlin has other plans. He is determined to find the lost Cauldron of Clyddno Eiddyn, one of the thirteen Treasures of Britain and a magical gift to the old Gods. Merlin fears the old gods will desert Britain and the land will be lost to Christianity; yet with the Cauldron, Merlin believes he can control the old gods and defeat the Christians. The Cauldron, however, lies in a land ruled by a ferocious Irish king, so Merlin needs warriors for his quest. To persuade Derfel and his men to join it, he gives Derfel a magic trinket with the power to prevent Ceinwyn’s and Lancelot’s marriage. But if Derfel uses it to win Ceinwyn’s love, all of Arthur's carefully constructed alliances may collapse.
Merlin’s obsession with the Cauldron, which comes to replace the story of the Holy Grail in Cornwell’s retelling of the Arthurian legend, is part of a larger conflict between the old pagan (Celtic) religion and the new Christian faith spreading across Britain – a well-portrayed theme in this series. This also provides some of the more thought provoking, and even disturbing, aspects of the story. For as whimsical and likeable as Merlin's character can be, his obsession with defeating Christianity and the things he's willing to do to achieve that goal can be troubling. Yet this was clearly one of Cornwell’s goals: to paint Marlin in distinct shades of gray instead of black or white.
Merlin’s quest for the Cauldron is only one part of the story, however. The rest concerns the growing threat of the Saxons, Camelot and the Round Table, and the love affairs and betrayals that are a hallmark of most Arthurian tales. Cornwell, however, puts a quite a twist on the latter aspect. This is not a story about a young, naive Guinevere falling for a chivalrous Sir Lancelot. No, Cornwell’s Guinevere is a fiercely strong and calculating woman, who, like Merlin, has her own plans in this novel, and the ramifications of those plans could decide the fate of Britain. This final element, in my view, makes Enemy of God as good as The Winter King, and it’s one of the reasons this book has stuck with me the longest of three in Cornwell’s masterful trilogy.