To many early Church Fathers, “mystical” signified the allegorical interpretation of scripture, especially the disclosing of Christ as the key to unlocking the secrets of the Old Testament. Scripture, Christologically interpreted, was the ground of all Christian thought, including mysticism, especially in the first centuries. Jesus was sometimes called the “mystical angel” whose entire life, death, resurrection, and glorification were understood as the truly mystical.
Eventually, Christians used the word “mystical” with respect to the sacraments, especially the Eucharist. The Fathers of the Church spoke of Christ’s “mystical Pasch,” the “mystical sacrifice of his Body and Blood,” and the “mystical bread and wine.” “Mystical waters,” of course, referred to baptism. The profoundest mysteries of the faith — such as the Trinity and Christ’s Divinity — were likewise deemed supremely “ineffable and mystical.” Especially in its formative stages, Christian mysticism was always both ecclesial, that is, realized only in and through the community, and scriptural, that is, tied to the spiritual, hidden, or mystical meaning of the sacred text.
— Harvey D. Egan, SJ, Soundings in the
Christian Mystical Tradition