Place and date of birth unknown; died 10 November, 461. Leo's pontificate, next to that of St. Gregory I, is the most significant and important in Christian antiquity. At a time when the Church was experiencing the greatest obstacles to her progress in consequence of the hastening disintegration of the Western Empire, while the Orient was profoundly agitated over dogmatic controversies, this great pope, with far-seeing sagacity and powerful hand, guided the destiny of the Roman and Universal Church. According to the "Liber Pontificalis" (ed. Mommsen, I, 101 sqq., ed. Duchesne, I, 238 sqq.), Leo was a native of Tuscany and his father's name was Quintianus. Our earliest certain historical information about Leo reveals him a deacon of the Roman Church under Pope Celestine I (422-32). Even during this period he was known outside of Rome, and had some relations with Gaul, since Cassianus in 430 or 431 wrote at Leo's suggestion his work "De Incarnatione Domini contra Nestorium" (Migne, P.L., L, 9 sqq.), prefacing it with a letter of dedication to Leo. About this time Cyril of Alexandria appealed to Rome against the pretensions of Bishop Juvenal of Jerusalem. From an assertion of Leo's in a letter of later date (ep. cxvi, ed. Ballerini, I, 1212; II, 1528), it is not very clear whether Cyril wrote to him in the capacity of Roman deacon, or to Pope Celestine. During the pontificate of Sixtus III (422-40), Leo was sent to Gaul by Emperor Valentinian III to settle a dispute and bring about a reconciliation between Aëtius, the chief military commander of the province, and the chief magistrate, Albinus. This commission is a proof of the great confidence placed in the clever and able deacon by the Imperial Court. Sixtus III died on 19 August, 440, while Leo was in Gaul, and the latter was chosen his successor. Returning to Rome, Leo was consecrated on 29 September of the same year, and governed the Roman Church for the next twenty-one years.
Leo's chief aim was to sustain the unity of the Church. Not long after his elevation to the Chair of Peter, he saw himself compelled to combat energetically the heresies which seriously threatened church unity even in the West. Leo had ascertained through Bishop Septimus of Altinum, that in Aquileia priests, deacons, and clerics, who had been adherents of Pelagius, were admitted to communion without an explicit abjuration of their heresy. The pope sharply censured this procedure, and directed that a provincial synod should be assembled in Aquileia, at which such persons were to be required to abjure Pelagianism publicly and to subscribe to an unequivocal confession of Faith (epp. i and ii). This zealous pastor waged war even more strenuously against Manichæism, inasmuch as its adherents, who had been driven from Africa by the Vandals, had settled in Rome, and had succeeded in establishing a secret Manichæan community there. The pope ordered the faithful to point out these heretics to the priests, and in 443, together with the senators and presbyters, conducted in person an investigation, in the course of which the leaders of the community were examined. In several sermons he emphatically warned the Christians of Rome to be on their guard against this reprehensible heresy, and repeatedly charged them to give information about its followers, their dwellings, acquaintances, and rendezvous (Sermo ix, 4, xvi, 4; xxiv, 4; xxxiv, 4 sq.; xlii, 4 sq.; lxxvi, 6). A number of Manichæans in Rome were converted and admitted to confession; others, who remained obdurate, were in obedience to imperial decrees banished from Rome by the civil magistrates. On 30 January, 444, the pope sent a letter to all the bishops of Italy, to which he appended the documents containing his proceedings against the Manichæans in Rome, and warned them to be on their guard and to take action against the followers of the sect (ep. vii). On 19 June, 445, Emperor Valentinian III issued, doubtless at the pope's instigation, a stern edict in which he established seven punishments for the Manichæans ("Epist. Leonis", ed. Ballerini, I, 626; ep. viii inter Leon. ep). Prosper of Aquitaine states in his "Chronicle" (ad an. 447; "Mon. Germ. hist. Auct. antiquissimi", IX, I, 341 sqq.) that, in consequence of Leo's energetic measures, the Manichæans were also driven out of the provinces, and even Oriental bishops emulated the pope's example in regard to this sect. In Spain the heresy of Priscillianism still survived, and for some time had been attracting fresh adherents. Bishop Turibius of Astorga became cognizant of this, and by extensive journeys collected minute information about the condition of the churches and the spread of Priscillianism. He compiled the errors of the heresy, wrote a refutation of the same, and sent these documents to several African bishops. He also sent a copy to the pope, whereupon the latter sent a lengthy letter to Turibius (ep. xv) in refutation of the errors of the Priscillianists. Leo at the same time ordered that a council of bishops belonging to the neighbouring provinces should be convened to institute a rigid enquiry, with the object of determining whether any of the bishops had become tainted with the poison of this heresy. Should any such be discovered, they were to be excommunicated without hesitation. The pope also addressed a similar letter to the bishops of the Spanish provinces, notifying them that a universal synod of all the chief pastors was to be summoned; if this should be found to be impossible, the bishops of Galicia at least should be assembled. These two synods were in fact held in Spain to deal with the points at issue (Hefele, "Konziliengesch." II, 2nd ed., pp. 306 sqq.).
The greatly disorganized ecclesiastical condition of certain countries, resulting from national migrations, demanded closer bonds between their episcopate and Rome for the better promotion of ecclesiastical life. Leo, with this object in view, determined to make use of the papal vicariate of the bishops of Arles for the province of Gaul for the creation of a centre for the Gallican episcopate in immediate union with Rome. In the beginning his efforts were greatly hampered by his conflict with St. Hilary, then Bishop of Arles. Even earlier, conflicts had arisen relative to the vicariate of the bishops of Arles and its privileges. Hilary made excessive use of his authority over other ecclesiastical provinces, and claimed that all bishops should be consecrated by him, instead of by their own metropolitan. When, for example, the complaint was raised that Bishop Celidonius of Besançon had been consecrated in violation of the canons—the grounds alleged being that he had, as a layman, married a widow, and, as a public officer, had given his consent to a death sentence—Hilary deposed him, and consecrated Importunus as his successor. Celidonius thereupon appealed to the pope and set out in person for Rome. About the same time Hilary, as if the see concerned had been vacant, consecrated another bishop to take the place of a certain Bishop Projectus, who was ill. Projectus recovered, however, and he too laid a complaint at Rome about the action of the Bishop of Arles. Hilary then went himself to Rome to justify his proceedings. The pope assembled a Roman synod (about 445) and, when the complaints brought against Celidonius could not be verified, reinstated the latter in his see. Projectus also received his bishopric again. Hilary returned to Arles before the synod was over; the pope deprived him of jurisdiction over the other Gallic provinces and of metropolitan rights over the province of Vienne, only allowing him to retain his Diocese of Arles.
These decisions were disclosed by Leo in a letter to the bishops of the Province of Vienne (ep. x). At the same time he sent them an edict of Valentinian III of 8 July, 445, in which the pope's measures in regard to St. Hilary were supported, and the primacy of the Bishop of Rome over the whole Church solemnly recognized "Epist. Leonis," ed. Ballerini, I, 642). On his return to his bishopric Hilary sought a reconciliation with the pope. After this there arose no further difficulties between these two saintly men and, after his death in 449, Hilary was declared by Leo as "beatæ memoriæ". To Bishop Ravennius, St. Hilary's successor in the see of Arles, and the bishops of that province, Leo addressed most cordial letters in 449 on the election of the new metropolitan (epp. xl, xli). When Ravennius consecrated a little later a new bishop to take the place of the deceased Bishop of Vaison, the Archbishop of Vienne, who was then in Rome, took exception to this action. The bishops of the province of Arles then wrote a joint letter to the pope, in which they begged him to restore to Ravennius the rights of which his predecessor Hilary had been deprived (ep. lxv inter ep. Leonis). In his reply dated 5 May, 450 (ep. lxvi), Leo acceded to their request. The Archbishop of Vienne was to retain only the suffragan Bishoprics of Valence, Tarentaise, Geneva, and Grenoble; all the other sees in the Province of Vienne were made subject to the Archbishop of Arles, who also became again the mediator between the Holy See and the whole Gallic episcopate. Leo transmitted to Ravennius (ep. lxvii), for communication to the other Gallican bishops, his celebrated letter to Flavian of Constantinople on the Incarnation. Ravennius thereupon convened a synod, at which forty-four chief pastors assembled. In their synodal letter of 451, they affirm that they accept the pope's letter as a symbol of faith (ep. xxix inter ep. Leonis). In his answer Leo speaks further of the condemnation of Nestorius (ep. cii). The Vicariate of Arles for a long time retained the position Leo had accorded it. Another papal vicariate was that of the bishops of Thessalonica, whose jurisdiction extended over Illyria. The special duty of this vicariate was to protect the rights of the Holy See over the district of Eastern Illyria, which belonged to the Eastern Empire. Leo bestowed the vicariate upon Bishop Anastasius of Thessalonica, just as Pope Siricius had formerly entrusted it to Bishop Anysius. The vicar was to consecrate the metropolitans, to assemble in a synod all bishops of the Province of Eastern Illyria, to oversee their administration of their office; but the most important matters were to be submitted to Rome (epp. v, vi, xiii). But Anastasius of Thessalonica used his authority in an arbitrary and despotic manner, so much so that he was severely reproved by Leo, who sent him fuller directions for the exercise of his office (ep. xiv).
In Leo's conception of his duties as supreme pastor, the maintenance of strict ecclesiastical discipline occupied a prominent place. This was particularly important at a time when the continual ravages of the barbarians were introducing disorder into allconditions of life, and the rules of morality were being seriously violated. Leo used his utmost energy in maintining this discipline, insisted on the exact observance of the ecclesiastical precepts, and did not hesitate to rebuke when necessary. Letters (ep. xvii) relative to these and other matters were sent to the different bishops of the Western Empire—e.g., to the bishops of the Italian provinces (epp. iv, xix, clxvi, clxviii), and to those of Sicily, who had tolerated deviations from the Roman Liturgy in the administration of Baptism (ep. xvi), and concerning other matters (ep. xvii). A very important disciplinary decree was sent to bishop Rusticus of Narbonne (ep. clxvii). Owing to the dominion of the Vandals in Latin North Africa, the position of the Church there had become extremely gloomy. Leo sent the Roman priest Potentius thither to inform himself about the exact condition, and to forward a report to Rome. On receiving this Leo sent a letter of detailed instructions to the episcopate of the province about the adjustment of numerous ecclesiastical and disciplinary questions (ep. xii). Leo also sent a letter to Dioscurus of Alexandria on 21 July, 445, urging him to the strict observance of the canons and discipline of the Roman Church (ep. ix). The primacy of the Roman Church was thus manifested under this pope in the most various and distinct ways. But it was especially in his interposition in the confusion of the Christological quarrels, which then so profoundly agitated Eastern Christendom, that Leo most brilliantly revealed himself the wise, learned, and energetic shepherd of the Church (see MONOPHYSITISM). From his first letter on this subject, written to Eutyches on 1 June, 448 (ep. xx), to his last letter written to the new orthodox Patriarch of Alexandria, Timotheus Salophaciolus, on 18 August, 460 (ep. clxxi), we cannot but admire the clear, positive, and systematic manner in which Leo, fortified by the primacy of the Holy See, took part in this difficult entanglement. For particulars refer to the articles: EUTYCHES ; SAINT FLAVIAN; ROBBER COUNCIL OF EPHESUS.
Eutyches appealed to the pope after he had been excommunicated by Flavian, Patriarch of Constantinople, on account of his Monophysite views. The pope, after investigating the disputed question, sent his sublime dogmatic letter to Flavian (ep. xxviii), concisely setting forth and confirming the doctrine of the Incarnation, and the union of the Divine and human natures in the one Person of Christ . In 449 the council, which was designated by Leo as the "Robber Synod", was held. Flavian and other powerful prelates of the East appealed to the pope. The latter sent urgent letters to Constantinople, particularly to Emperor Theodosius II and Empress Pulcheria, urging them to convene a general council in order to restore peace to the Church. To the same end he used his influence with the Western emperor, Valentinian III, and his mother Galla Placidia, especially during their visit to Rome in 450. This general council was held in Chalcedon in 451 under Marcian, the successor of Theodosius. It solemnly accepted Leo's dogmatical epistle to Flavian as an expression of the Catholic Faith concerning the Person of Christ. The pope confirmed the decrees of the Council after eliminating the canon, which elevated the Patriarchate of Constantinople, while diminishing the rights of the ancient Oriental patriarchs. On 21 March, 453, Leo issued a circular letter confirming his dogmatic definition (ep. cxiv). Through the mediation of Bishop Julian of Cos, who was at that time the papal ambassador in Constantinople, the pope tried to protect further ecclesiastical interests in the Orient. He persuaded the new Emperor of Constantinople, Leo I, to remove the heretical and irregular patriarch, Timotheus Ailurus, from the See of Alexandria. A new and orthodox patriarch, Timotheus Salophaciolus, was chosen to fill his place, and received the congratulations of the pope in the last letter which Leo ever sent to the Orient.
In his far-reaching pastoral care of the Universal Church, in the West and in the East, the pope never neglected the domestic interests of the Church at Rome. When Northern Italy had been devastated by Attila, Leo by a personal encounter with the King of the Huns prevented him from marching upon Rome. At the emperor's wish, Leo, accompanied by the Consul Avienus and the Prefect Trigetius, went in 452 to Upper Italy, and met Attila at Mincio in the vicinity of Mantua, obtaining from him the promise that he would withdraw from Italy and negotiate peace with the emperor. The pope also succeeded in obtaining another great favour for the inhabitants of Rome. When in 455 the city was captured by the Vandals under Genseric, although for a fortnight the town had been plundered, Leo's intercession obtained a promise that the city should not be injured and that the lives of the inhabitants should be spared. These incidents show the highmoral authority enjoyed by the pope, manifested even in temporal affairs. Leo was always on terms of intimacy with the Western Imperial Court. In 450 Emperor Valentinian III visited Rome, accompanied by his wife Eudoxia and his mother Galla Placidia. On the feast of Cathedra Petri (22 February), the Imperial family with their brilliant retinue took part in the solemn services at St. Peter's, upon which occasion the pope delivered an impressive sermon. Leo was also active in building and restoring churches. He built a basilica over the grave of Pope Cornelius in the Via Appia. The roof of St. Paul's without the Walls having been destroyed by lightning, he had it replaced, and undertook other improvements in the basilica. He persuaded Empress Galla Placidia, as seen from the inscription, to have executed the great mosaic of the Arch of Triumph, which has survived to our day. Leo also restored St. Peter's on the Vatican. During his pontificate a pious Roman lady, named Demetria, erected on her property on the Via Appia a basilica in honour of St. Stephen, the ruins of which have been excavated.
Leo was no less active in the spiritual elevation of the Roman congregations, and his sermons, of which ninety-six genuine examples have been preserved, are remarkable for their profundity, clearness of diction, and elevated style. The first five of these, which were delivered on the anniversaries of his consecration, manifest his lofty conception of the dignity of his office, as well as his thorough conviction of the primacy of the Bishop of Rome, shown forth in so outspoken and decisive a manner by his whole activity as supreme pastor. Of his letters, which are of great importance for church history, 143 have come down to us: we also possess thirty which were sent to him. The so-called "Sacramentarium Leonianum" is a collection of orations and prefaces of theMass, prepared in the second half of the sixth century. Leo died on 10 November, 461, and was buried in the vestibule of St. Peter's on the Vatican. In 688 Pope Sergius had his remains transferred to the basilica itself, and a special altar erected over them. They rest today in St. Peter's, beneath the altar specially dedicated to St. Leo. In 1754 Benedict XIV exalted him to the dignity of Doctor of the Church (doctor ecclesiæ). In the Latin Church the feast day of the great pope is held on 11 April, and in the Eastern Church on 18 February.