Short answers, pulled from the story.
John of the Cross was a Spanish Catholic priest, Carmelite friar, and mystic born on the 24th of June 1542 in Fontiveros, Old Castile. He is one of 38 Doctors of the Church, declared so by Pope Pius XI in 1926, and is considered one of the foremost poets in Spanish. His writings on the spiritual life, including the Spiritual Canticle and Dark Night of the Soul, are regarded as the summit of mystical Christian literature.
The Dark Night of the Soul is a poem by John of the Cross, likely written in 1578 or 1579, that narrates the journey of the soul from its bodily home toward union with God through successive stages of hardship and detachment. John wrote a commentary on its first two stanzas and the opening line of the third stanza in 1584-5. The phrase "dark night" represents the difficulties encountered in separating from worldly attachments before reaching spiritual union.
On the night of the 2nd of December 1577, Carmelites opposed to reform seized John and transported him to the Carmelite monastery in Toledo, a community of 40 friars. He was kept in a tiny cell, subjected to public lashings at least weekly, given only water, bread, and scraps of salt fish, and allowed no change of clothing. He escaped eight months later on the 15th of August 1578 by prying open his cell door and slipping through a small window.
The Spiritual Canticle is an eclogue by John of the Cross in which a bride, representing the soul, searches for and is reunited with her bridegroom, representing Jesus Christ. Its first 31 stanzas were composed in 1578 while John was imprisoned in Toledo. Two versions survive today, one with 39 stanzas and one with 40; the first commentary on the poem was written in 1584 at the request of Madre Ana de Jesús, then prioress in Granada.
John of the Cross was canonized by Pope Benedict XIII in 1726. He was declared a Doctor of the Church by Pope Pius XI in 1926, following consultation with Reginald Garrigou-Lagrange, professor of philosophy and theology at the Pontifical University of Saint Thomas Aquinas in Rome. He is also known as the mystical doctor.
The Discalced Carmelites were a reformed branch of the Carmelite Order, founded to restore the stricter observances of the original Carmelite Primitive Rule of 1209, which included fasting, silence, solitude, and going without shoes. John of the Cross helped establish their first monastery for friars at Duruelo on the 28th of November 1568, alongside Teresa of Ávila. The order was formally recognized as separate by Pope Gregory XIII in a decree signed on the 22nd of June 1580.