Latin Church Father (c. 400-461) and Pope Leo I (440-461). Born probably in Tuscany; deacon of the Roman church before his election as bishop of Rome in 440. Leo's episcopate was dominated by two major concerns: (1) the substantive Christological controversies of the mid-fifth century leading to the Council of Chalcedon (451); (2) the military and political crises of the mid-fifth century Western Roman Empire, including the famous meeting with Attila the Hun outside Mantua (452, where Leo successfully persuaded Attila to withdraw from the planned attack on Rome) and the partial negotiations with Geiseric the Vandal (455, sparing Rome from total destruction during the Vandal sack). Leo's principal theological contribution is the Tome of Leo (449, formally Epistula CXXIV ad Flavianum, his letter to Flavian of Constantinople), a substantive theological statement on the orthodox Christology against the Monophysite (one-nature) heresy of Eutyches. The Tome articulates the orthodox position that Christ is one Person in two complete natures (divine and human), the natures preserved without confusion, change, division, or separation; the Tome was formally received and integrated into the Definition of Chalcedon (451) as one of the substantive theological foundations of the council's orthodox Christological settlement (the Chalcedonian Definition explicitly incorporates the Tome's language). Leo also produced 96 surviving sermons and 173 surviving letters; the sermons cover the liturgical year, doctrinal topics, and pastoral matters. Leo is one of only two popes called the Great (the other is Gregory the Great, 590-604). The patriarchal-Reformed reader receives Leo's substantive Christological work at Chalcedon as foundational to orthodox confession; the Roman-Catholic claims of papal jurisdictional supremacy that emerged partly from Leo's writings are engaged with appropriate Reformed discernment.