From her earliest years, Catherine of Siena (1347-80) was griped by the mystery of God incarnate. This was the beginning of a life-long love story, a life-long conversion in which Christ spoke to Catherine and she communicated the truth of his being to the world of her time. Creation itself, she says, is an act of love, and Christ is the bridge by which human beings come to realize their full humanity, their inmost nature, which is to love.

Mystic, champion of the poor and Church reformer, Catherine was declared a Doctor of the Church in 1970. She was unable to write until she was...