In Catholic teaching, such states do not come about even with the ordinary aid of Divine grace. Mystical theology, then, comprises among its subjects all extraordinary forms of prayer, the higher forms of contemplation in all their varieties or gradations, private revelations, visions, and the union growing out of these between God and the soul, known as the mystical union. As the science of all that is extraordinary in the relations between the Divinity and the human spirit, mystical theology is the complement of ascetical theology, which treats of Christian perfection and of its acquisition by the practice of virtue, particularly by the observance of the counsels.
What strictly comes within the province of mystical theology is the study of the processes of active and passive purification through which a soul must pass to reach the mystical union. Although the active processes are also treated to some extent in ascetical theology, they require special study inasmuch as they lead to contemplation. They comprise: purity of conscience, or aversion even to the slightest sin; purity of heart, the heart being taken as the symbol of the affections, which to be pure must be free of attachments to anything that does not lead to God; purity of the spirit, i. e. of the imagination and memory; and purity of action. It is to these processes that the well-known term "night" is applied by Discalced Carmelite reformer St. John of the Cross, since they imply three things which are as night to the soul in so far as they are beyond or contrary to its own lights, viz., the privation of pleasure, faith as substituted for human knowledge, and God as incomprehensible, or darkness, to the unaided soul. Passive purifications are the trials encountered by souls in preparation for contemplation, known as desolation, or dryness, and weariness. As they proceed sometimes from God and sometimes may be produced by the Evil Spirit, rules for the discernment of spirits are set down to enable directors to determine their source and to apply proper means of relief, especially should it happen that the action of the Evil One tends to possession or obsession.
These passive purifications affect the soul when every other object of contemplation is withdrawn from it, except its own sins, defects, frailties, which are revealed to it in all their enormity. They put the soul in the "obscure night", as St. John of the Cross calls it, or in the "great desolation", to use the phrase of Father Baker. In this state the soul experiences many trials and temptations, even to infidelity and despair, all of which are expressed in the peculiar terminology of writers on mystical theology, as well as the fruits derived from resisting them. Chief among these fruits is the purification of love, until the soul is so inflamed with love of God that it feels as if wounded and languishes with the desire to love Him still more intensely. The first difficulty mystical writers encounter in their treatises on contemplation is the proper terminology for its degrees, or the classification of the experiences of the soul as it advances in the mystical union with God effected by this extraordinary form of prayer. Ribet in "La Mystique Divine" has a chapter (x) on this subject, and the present writer treats it in chapter xxix of his "Grace of Interior Prayer" (tr. of the sixth edition). Giovanni Battista Scaramelli follows this order: the prayer of recollection; the prayer of spiritual silence; the prayer of quiet; the inebriation of love; the spiritual sleep; the anguish of love; the mystical union of love, and its degrees from simple to perfect union and spiritual marriage. In this union the soul experiences various spiritual impressions, which mystical writers try to describe in the terminology used to describe sense impressions, as if the soul could see, hear, touch, or enjoy the savour or odour of the Divinity. Ecstatic union with God is a further degree of prayer. This and the state of rapture require careful observation to be sure that the Evil One has no share in them. Here again mystical writers treat at length the deceits, snares, and other arts practised by the Evil One to lead souls astray in the quest for the mystical union. Finally, contemplation leads to a union so intimate and so strong that it can be expressed only by the terms "spiritual marriage". The article on contemplation describes the characteristics of the mystical union effected by contemplation. No treatise of mystical theology is complete without chapters on miracles, prophecies, revelations, visions, all of which have been treated under their respective headings.
As for the history or development of mysticism, it is as difficult to record as a history of the experiences of the human soul. The most that can be done is to follow its literature, mindful that the most extraordinary mystical experiences defy expression in human speech, and that God, the Author of mystical states, acts upon souls when and as He wills, so that there can be no question of what we could consider a logical or chronological development of mysticism as a science. Still, it is possible to review what mystical writers have said at certain periods, and especially what the Carmelite saint, Teresa of Avila, did to treat for the first time mystical phenomena as a science. Before her, mystics were concerned principally with ecstasies, visions, and revelations; she was the first to attempt a scientific analysis of the process of mystical union brought about by contemplation. As the contribution to the science and history of mystical theology by each of the writers in the following list has been sufficiently noted in the articles on them, it will suffice here to mention the titles of some of their characteristic works.
De Theologia Mystica is a treatise of Pseudo-Dionysius the Areopagite discussing the transcendent nature of God. The writings of Dionysius the Pseudo-Areopagite did not reach the West until about 824, when they were sent to Louis the Pious by Michael the Stammerer, Emperor of Constantinople: "Opera".
A number of later works on the topic have the same title: