Mystical union may be described as the relationship between a person and God in the highest degrees of the mystical life. Ordinarily, mystical union is said to have three stages: prayer of union, prayer of ecstatic union, and prayer of transforming union (mystical marriage).
In the prayer of union the soul is deeply aware of God's presence. All the internal powers of the soul, including the memory and imagination, are captivated and occupied with God. This union, usually of short duration, is marked by the absence of distractions, and the certainty of being deeply united to God.
The prayer of ecstatic union differs from the prayer of union in that the external senses are also suspended or captivated. As the intensity of the mystical union grows, it becomes so great that the body cannot withstand it and so falls into ecstasy. In this union the Holy Spirit, acting through His gifts, so intimately and ardently unites the soul to God that the natural weakness of the subject cannot withstand the intensity of the light and love communicated. The soul falls into ecstasy, and this causes the body to experience an alienation of the senses.
In the prayer of transforming union (mystical marriage) there is a complete transformation of the soul into the Beloved. God gives Himself to the soul and the soul gives itself to God in a certain consummation of divine love, so that the soul shares in God's life as fully as is possible in this life. This union is more or less permanent; the soul is more conscious than ever of the Blessed Trinity. The soul is absorbed in seeking the honor of God, eagerly desiring to undertake anything or suffer anything that God may will.
Bibliography: teresa of avila, Interior Castle; in Complete Works, ed. silverio de santa teresa and e. a. peers, 3 v. (New York 1946) v. 2. john of the cross, The Living Flame of Love, tr. d. lewis (New York 1912). a. royo, The Theology of Christian Perfection, tr. and ed. j. aumann (Dubuque 1962).