In the 1480s Dominican humanist Filippo de' Barbieri published an illustration of a supposedly ancient female seer called the 'Sybilla Chimica', whose prophetic text repeated the words of the ninth-century astrologer Abu Ma'shar. In tracing the origins of Barbieri's astrological Sibyl, this article examines three sometimes interlocking traditions: the attribution of an ante-diluvian history to the science of the stars, the assertion of astrology's origins in divine revelation, and the belief in the ancient Sibyls' predictions of the birth of Christ and other Christian truths. Medieval authors from the twelfth century on began to cite these traditions together, thereby simultaneously authorizing the use of astrology to predict religious changes and blurring the categories of natural and supernatural as applied to human understanding. This blending of astrology and prophecy appears notably in works by such authors as John of Paris, John of Legnano, Johannes Lichtenberger, and Marsilio Ficino. Ultimately the trajectory that produced Barbieri's astrological Sibyl would lead to a wave of astrological apocalyptic predictions in the sixteenth and seventeenth centuries, as well as to the harnessing of astrology for the defense of the faith in the form of an astrological natural theology, sacralizing science as well as nature.