She says faith is like crystal for two reasons: first, because it concerns Christ, her Bridegroom; second, because it has the characteristics of crystal, being pure in its truths, strong, clear, and cleansed of errors and natural forms. And she calls it a spring because from it the waters of all spiritual goods flow into the soul. Christ our Lord, speaking with the Samaritan woman, called faith a spring, declaring that in those who believe in him he would make a fountain whose waters would leap up unto life everlasting.
(ST. JOHN OF THE CROSS, Spiritual Canticle 12, 3.)