Gertrud the Great (1256-1302) entered the monastery of Helfta in eastern Germany as a child oblate. At the age of twenty-five she underwent a conversion that led to a series of visionary experiences. These centered on "the divine loving-kindness," which she perceived as expressed through and symbolized by Christ's divine Heart. Some of these experiences she recorded in Latin "with her own hand," in what became Book 2 of The Herald of God's Loving-Kindness.

Books 1, 3, 4, and 5 were written down by another nun, a close confidant of the saint, now often known as "Sister N." Book...