In the Abbey of Viboldone, in suburban Milan, there is a medieval Trinitarian sinopia that never before has received an in-depth analysis. This paper proposes that the sinopia is related to a group known as the Guglielmites, prosecuted in this region of Milan in 1300. The Guglielmites believed that a woman named Guglielma was the Holy Spirit, the third person in the Christian Trinity, come to earth incarnate in female form. The sinopia appears to portray a female Holy Spirit, and the history of the Abbey in which it is located suggests some intriguing relationships with this heretical group.