Catechism of the Catholic Church §696 — excerpt
Fire. While water signifies birth and the fruitfulness of life given in the Holy Spirit, fire symbolizes the transforming energy of the Holy Spirit's actions.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. John of the Cross, the Living Flame of Love, in the Collected Works of St. John of the Cross, tr. K. Kavanaugh, OCD, · O. Rodriguez, OCD (Washington DC: Institute of Carmelite Studies, 1979), 577 ff