MacCulloch’s work emphasizes that Celtic mythology is a "living" mythology. It is defined by its , its reverence for nature , and its persistence through oral tradition and later literature. He successfully argues that the Celtic spirit is found not in rigid dogma, but in the magical intersection of the natural and supernatural worlds.
MacCulloch organizes the vast Celtic pantheon by focusing on regional cycles. He argues that while there is no single "Celtic Bible," there are recurring archetypes across different cultures:
Many goddesses were repurposed as Christian saints (e.g., Brigid) or diminished into the "Fair Folk" of later folklore.