Teresa de Cepeda y Ahumada was born into the gentry of Ávila, the daughter of a converso (Jewish-Christian) merchant. She lost her mother at thirteen, ran away as a child with her brother to seek martyrdom among the Moors, and entered the Carmel of the Incarnation in Ávila at twenty against her father's wishes.
Her first twenty years in religion were ordinary, even slack — she would later call them 'twenty years on a stormy sea, neither in mortal sin nor in true devotion.' At about forty, gazing at a wounded Christ statue brought for Holy Week, she felt herself broken open. From that moment her prayer deepened rapidly, and the great mystical phenomena began.
Despite illness, the Spanish Inquisition's suspicion, and the bitter opposition of the unreformed Carmelites, she founded seventeen new monasteries of the stricter 'Discalced' (barefoot) reform — and the male branch with the help of St. John of the Cross. She wrote four spiritual classics under obedience: her Autobiography, The Way of Perfection, The Foundations, and the supreme work, The Interior Castle (1577).
She died on the night of October 4–5, 1582 — on the very day the Gregorian calendar reform skipped to October 15, so the Church marks her death on that date. She was canonised in 1622 and, in 1970, became the first woman declared a Doctor of the Church.