Ordained a Carmelite priest at 25 (1567), John met St. Teresa of Jesus (Avila) and like her vowed himself to the primitive Rule of the Carmelites. As partner with Teresa and in his own right, John engaged in the work of reform, and came to experience the price of reform: increasing opposition, misunderstanding, persecution, imprisonment. He came to know the cross acutely—to experience the dying of Jesus—as he sat month after month in his dark, damp, narrow cell with only his God!
Yet, the paradox! In this dying of imprisonment John came to life, uttering poetry. In the darkness of the dungeon, John's spirit came into the Light. There are many mystics, many poets; John is unique as mystic-poet, expressing in his prison-cross the ecstasy of mystical union with God in the Spiritual Canticle.
But as agony leads to ecstasy, so John had his Ascent to Mt. Carmel, as he named it in his prose masterpiece. As man-Christian-Carmelite, he experienced in himself this purifying ascent; as spiritual director, he sensed it in others; as psychologist-theologian, he described and analyzed it in his prose writings. His prose works are outstanding in underscoring the cost of discipleship, the path of union with God: rigorous discipline, abandonment, purification. Uniquely and strongly John underlines the gospel paradox: The cross leads to resurrection, agony to ecstasy, darkness to light, abandonment to possession, denial to self to union with God. If you want to save your life, you must lose it. John is truly "of the Cross." He died at 49—a life short, but full.
Thomas Merton said of John: "Just as we can never separate asceticism from mysticism, so in St. John of the Cross we find darkness and light, suffering and joy, sacrifice and love united together so closely that they seem at times to be identified."
--From Saint of the Day by St. Anthony Messenger Press
Fr. Karol Wojtyla (later Pope John Paul II) considered entering the Discalced Carmelite Monastery as a young man, but in 1945 gave up that idea in order to become a parish priest. Fr. Wojtyla was always inspired by St. John of the Cross, so much so, that he taught himself Spanish with the goal to read St. John of the Cross in the original language written.
--from Witness to Hope by George Weigel