probably a Syrian monk who, known only by his pseudonym, wrote a series of Greek treatises and letters for the purpose of uniting Neoplatonic philosophy with Christian theology and mystical experience. These writings established a definite Neoplatonic trend in a large segment of medieval Christian doctrine and spirituality—especially in the Western Latin Church—that has determined facets of its religious and devotional character to the present time. Historical research has been unable to identify the author, who, having assumed the name of the New Testament convert of St. Paul (Acts 17:34), could have been one of several Christian writers familiar with the Neoplatonic system of the 5th-century Athenian Proclus. In the 9th century Dionysius was confused with St. Denis of France; but this was disproved in the 12th century by Peter Abelard.

The treatises “On the Divine Names,” “On Mystical Theology,” “On the Celestial Hierarchy,” and “On the Ecclesiastical Hierarchy” comprise the bulk of the Dionysian corpus of writings, supplemented with 10 letters affecting a 1st-century primitive Christian atmosphere. Their doctrinal content forms a complete theology, covering the Trinity and angelic world, the incarnation and redemption, and the last things, and provides a symbolic and mystical explanation of all that is. The system is essentially dialectical, or “crisis” (from the Greek word meaning “crossroads, decision”), theology—i.e., the simultaneous affirmation and denial of paradox in any statement or concept relative to God. God's transcendence above all rational comprehension and categorical knowledge ultimately reduces any expression of the divinity to polar pairs of contraries: grace and judgment, freedom and necessity, being and nonbeing, time and eternity. The incarnation of the Word, or Son of God, in Christ, consequently, was the expression in the universe of the inexpressible, whereby the One enters into the world of multiplicity. Still, the human intellect can apply to God positive, analogous terms or names such as The Good, Unity, Trinity, Beauty, Love, Being, Life, Wisdom, or Intelligence, assuming that these are limited forms of communicating the incommunicable.

The “Divine Names” and “Mystical Theology” treat the nature and effects of contemplative prayer—the disciplined abandonment of senses and intelligible forms to prepare for the immediate experience of “light from the divine darkness” and ecstatic union—in a manner and scope that make them indispensable to the history of Christian theology and piety. His treatises on the hierarchies, wherein he theorized that all that exists—the form of Christian society, the stages of prayer, and the angelic world—is structured as triads that are the images of the eternal Trinity, introduced a new meaning for the term hierarchy.

The 9th-century Irish philosopher-humanist John Scotus Erigena made a Latin translation of his writings, and the 12th- and 13th-century Scholastics Hugh of Saint-Victor (Paris), Albertus Magnus, and Thomas Aquinas wrote commentaries on them. The 14th- and 15th-century Rhineland and Flemish mystics, and the 16th-century Spanish mystics all were influenced by Dionysian thought. Writers of the Greek and Eastern churches, already sympathetic toward Platonic thought, simply absorbed the Dionysian corpus in their theologies as one element among others of this intellectual school. Such syntheses were effected by Gregory of Nazianzus and other 4th-century Cappadocian theologians, the 7th-century résumé of Maximus the Confessor, and the works of the 14th-century mystic Gregory Palamas.