How we know. Catherine's life is documented in unusual detail for a fourteenth-century figure. She herself dictated The Dialogue and composed nearly four hundred letters in her own voice; Raymond of Capua, her confessor and spiritual director for the last six years of her life, wrote the principal hagiography Legenda Major within fifteen years of her death. The first-hand record is dense, the principal hagiography is contemporary, and where Raymond's account leans toward the marvelous it can frequently be cross-checked against the letters themselves. Catherine is one of the medieval saints about whom we know the most, and about whom we know most reliably.

Catherine Benincasa was born on March 25, 1347, in Siena, the daughter of a wool dyer and one of twenty-five children. From childhood she reported mystical experiences and visions. Around the age of seven, she experienced a vision of Christ and resolved to dedicate herself to a contemplative life. Against her family's wishes for marriage, at sixteen she joined the Dominican Third Order (the Mantellate, the lay tertiaries), committing to celibacy, prayer, and service. She lived as a tertiary in her father's house, not in a convent, observing severe ascetical practices: extreme fasting and extended vigils.

Beginning in 1370 she engaged in active mission work, serving plague victims in Siena's hospitals and prisons and gaining a reputation for spiritual counsel and miraculous healings. During this period she dictated 383 surviving letters to popes, cardinals, nobles, religious leaders, and spiritual seekers, addressing both personal direction and the major ecclesiastical issues of her age. The letters are remarkable for their spiritual intensity, political insight, and unflinching criticism of church corruption. Between 1377 and 1378 she composed her major theological work, The Dialogue (Il Libro della Divina Dottrina), a mystical exchange between the Soul and God on sin, redemption, discretion, virtue, and the Christian life. One of its enduring images is Christ as the bridge between earth and heaven through his wounds: "Out of mercy you have washed us in his Blood, out of mercy you have wished to converse with creatures. O crazed with love! It did not suffice for you to take flesh, but you also wished to die!… O mercy! My heart drowns in thinking of you: for no matter where I turn to think, I find only mercy" (chapter 30).

In 1376, at twenty-nine, Catherine turned much of her attention toward political activism, particularly toward restoring the papacy to Rome from its exile in Avignon. The roots of that exile lay seventy-one years earlier, when King Philip IV of France and Pope Boniface VIII clashed over control of the French clergy and church revenue. Boniface issued Unam Sanctam in 1302, asserting that submission to the pope was necessary for salvation; Philip's soldiers assaulted him at Anagni in 1303, and Boniface died a few weeks later. In the climate of political violence that followed, his successor Clement V was enthroned in Lyon and eventually settled the papal court in Avignon.

Into this drama Catherine traveled to Avignon, where Pope Gregory XI was then reigning. She urged him to return to Rome, and her arguments were less political than theological and moral. She urged Gregory to keep a vow he had reportedly made to return the papacy; she argued for Rome as the designated place for Peter's chair; she insisted that the pope must lead clerical reform from Rome itself; and she warned that cowardice in the face of political pressure was a sin. She told him directly that he was failing through fear of men rather than fear of God, calling on him to become a "manly man, free from any fear or fleshly love toward yourself, or toward any creature related to you in the flesh." If he would not exercise his authority, she said, he should resign: "you should use your virtue and power: and if you are not willing to use it, it would be better for you to resign what you have assumed; more honor to God and health to your soul would it be." Pope Benedict XVI later described her as a woman who "impels us to walk courageously toward holiness to be ever more fully disciples of the Lord."

Her spiritual authority, boldness, and moral conviction (unusual for anyone, and in her era especially for a woman without formal theological training) impressed the pope. Gregory XI departed Avignon in late 1376 and arrived in Rome in January 1377. Catherine's intervention was celebrated as a decisive achievement in ending the so-called "Babylonian Captivity of the Church." After Gregory's death in March 1378, rival claimants arose and the Western Schism began. Catherine supported Urban VI, the Roman pope, against the Avignon antipopes; she spent her final years writing letters and negotiating on behalf of the unified papacy. She died on April 29, 1380, in Rome, aged thirty-three, evidently from years of extreme ascetical exhaustion. She was canonized by Pope Pius II on June 29, 1461, and declared a Doctor of the Church by Pope Paul VI on October 4, 1970, alongside Teresa of Ávila; the first women to receive the title.