The Flowing Light of the Godhead was composed over about thirty years and it does not look like a treatise. It is assembled from short pieces in many forms: dialogues between the Soul and Love, or the Soul and God, verse, prayer, vision, allegory, satire, and stretches of straightforward complaint. The material is arranged in seven books, the last written when Mechthild was old and going blind and dictating.
Its historical importance is partly linguistic. She wrote in Middle Low German, a language that in the middle of the thirteenth century had almost no theological vocabulary, so where a Latin writer would reach for an established term she has to build an image instead. That constraint produces the book's characteristic manner: God as a flowing light, the soul as a bride at a dance, desire described in the erotic register of the Song of Songs and pressed much further than a Latin commentator would have pressed it.
She is also unsparing about the church of her day. Corrupt clergy are attacked directly and by name of office, canons are compared to goats, and she reports being warned that the book would be burned. She answers, in the text, that she wrote because her confessor commanded it and because she was told to by God, which is the standard defence a medieval woman writer had available and which she deploys with some skill.