The three major denominations of Catholicism, Eastern Orthodoxy and Oriental Orthodoxy all encourage a development of a profound mystical unity with God.
Christian mysticism is the tradition of mystical practices and mystical theology within Christianity which "concerns the preparation [of the person] for, the consciousness of, and the effect of [...] a direct and transformative presence of God" or Divine love. Until the sixth century the practice of what is now called mysticism was referred to by the term contemplatio, c.q. theoria, from contemplatio (Latin; Greek θεωρία, theoria), “looking at", "gazing at", "being aware of" God or the Divine. Christianity took up the use of both the Greek (theoria) and Latin (contemplatio, contemplation) terminology to describe various forms of prayer and the process of coming to know God.
Contemplative practices range from simple prayerful meditation of Holy Scripture (i.e. Lectio Divina) to contemplation on the presence of God, resulting in theosis (spiritual union with God) and ecstatic visions of the soul's mystical union with God. Three stages are discerned in contemplative practice, namely catharsis (purification), contemplation proper, and the vision of God.
Contemplative practices have a prominent place in the Eastern Orthodoxy and Oriental Orthodoxy, and have gained a renewed interest in western Christianity.
Mysticism
"Mysticism" is derived from the Greek μύω, meaning "to conceal," and its derivative μυστικός, mystikos, meaning "an initiate." In the Hellenistic world, a "mystikos" was an initiate of a mystery religion. "Mystical" referred to secret religious rituals and use of the word lacked any direct references to the transcendental.
In early Christianity the term mystikos referred to three dimensions, which soon became intertwined, namely the biblical, the liturgical and the spiritual or contemplative. The biblical dimension refers to "hidden" or allegorical interpretations of Scriptures. The liturgical dimension refers to the liturgical mystery of the Eucharist, the presence of Christ at the Eucharist. The third dimension is the contemplative or experiential knowledge of God.
Eastern Orthodox Christianity
Eastern Christianity has preserved a mystical emphasis in its theology and retains in hesychasm a tradition of mystical prayer dating back to Christianity's beginnings. Hesychasm concerns a spiritual transformation of the egoic self, the following of a path designed to produce more fully realized human persons, "created in the Image and Likeness of God" and as such, living in harmonious communion with God, the Church[citation needed], the rest of the world, and all creation, including oneself. The Eastern Christian tradition speaks of this transformation in terms of theosis or divinization, perhaps best summed up by an ancient aphorism usually attributed to Athanasius of Alexandria: "God became human so that man might become god."
According to John Romanides, in the teachings of Eastern Orthodox Christianity the quintessential purpose and goal of the Christian life is to attain theosis or 'deification', understood as 'likeness to' or 'union with' God. Theosis is expressed as "Being, union with God" and having a relationship or synergy between God and man. God is the Kingdom of Heaven.
Theosis or unity with God is obtained by engaging in contemplative prayer, the first stage of theoria, which results from the cultivation of watchfulness (Gk: nepsis). In theoria, one comes to see or "behold" God or "uncreated light," a grace which is "uncreated." In the Eastern Christian traditions, theoria is the most critical component needed for a person to be considered a theologian; however it is not necessary for one's salvation. An experience of God is necessary to the spiritual and mental health of every created thing, including human beings. Knowledge of God is not intellectual, but existential. According to eastern theologian Andrew Louth, the purpose of theology as a science is to prepare for contemplation, rather than theology being the purpose of contemplation.
Latin Catholic mysticism
Contemplatio
In the Latin Church terms derived from the Latin word contemplatio such as, in English, "contemplation" are generally used in languages largely derived from Latin, rather than the Greek term theoria. The equivalence of the Latin and Greek terms was noted by John Cassian, whose writings influenced the whole of Western monasticism, in his Conferences. However, Catholic writers do sometimes use the Greek term.
Middle Ages
The Early Middle Ages in the West includes the work of Gregory the Great and Bede, as well as developments in Celtic Christianity and Anglo-Saxon Christianity, and comes to fulfillment in the work of Johannes Scotus Eriugena and the Carolingian Renaissance.
The High Middle Ages saw a flourishing of mystical practice and theorization corresponding to the flourishing of new monastic orders, with such figures as Guigo II, Hildegard of Bingen, Bernard of Clairvaux, the Victorines, all coming from different orders, as well as the first real flowering of popular piety among the laypeople.
The Late Middle Ages saw the clash between the Dominican and Franciscan schools of thought, which was also a conflict between two different mystical theologies: on the one hand that of Dominic de Guzmán and on the other that of Francis of Assisi, Anthony of Padua, Bonaventure, Jacopone da Todi, Angela of Foligno. Moreover, there was the growth of groups of mystics centered on geographic regions: the Beguines, such as Mechthild of Magdeburg and Hadewijch (among others); the Rhenish-Flemish mystics Meister Eckhart, Johannes Tauler, Henry Suso, and John of Ruysbroeck; and the English mystics Richard Rolle, Walter Hilton and Julian of Norwich. This period also saw such individuals as Catherine of Siena and Catherine of Genoa, the Devotio Moderna, and such books as the Theologia Germanica, The Cloud of Unknowing and The Imitation of Christ.