Her subject was the transformation of the soul through agape (Christian love, as distinct from physical or emotional love). Using poetry and prose, she outlined seven stages of the soul on its path to Union with God.
The more important issue with the book was that it expressed ideas similar to those of the Brethren of the Free Spirit, a loose movement in the Low Countries between the 13th and 15th centuries. Some of those ideas were that Christ, the church, and the sacraments were not necessary for salvation, because the soul could be perfected on its own by connecting to God's love. In fact, the perfection of the soul meant that the soul and God were one.
Her book was copied and spread among Beguines and others. Authorities rounded up all the copies they could find, burned them, and then imprisoned Marguerite. She spent a year and a half imprisoned, speaking to no one. Finally, a trial was held, during which she refused to renounce the ideas expressed in the Mirror, or to promise to never express them again.
Her refusal led to her burning at the stake on 1 June 1310. Although it remained popular after the trial, and was widely circulated, Le Mirouer des simples âmes was known to modern times only through the record of her trial. In 1911, a purchase of old manuscripts by the British Library from a private collector turned up an English translation made in the 15th century. Three other manuscripts were eventually found, one in Latin. Various translations have been published since then.
There was a time when the copies circulated after her death were wrongly attributed to another author, John van Ruysbroeck. What made him a likely candidate for this mistake? We will meet him tomorrow.