Leo the Great, a Roman aristocrat, was Pope from 440 to 461. He is the first great Pope we know much about, and even sometimes assigned the title "first Pope". He stopped the invasion of Italy by Atilla the Hun in 452 by his moral suasion, was a great theologian in his own right, and was a leading figure in the centralization of the government of the Church.
Zeal for orthodoxy
An uncompromising foe of heresy, Leo found that in the diocese of Aquileia, Pelagians were received into church communion without formal repudiation of their errors; he wrote to rebuke this culpable negligence, and required a solemn abjuration before a synod.
Manicheans fleeing before the Vandals had come to Rome in 439 and secretly organized there; Leo learned of this around 443, and proceeded against them by holding a public debate with their representatives, burning their books, and warning the Roman Christians against them. His efforts led to the edict of Valentinian III against them (June 19, 445).
Nor was his attitude less decided against the Priscillianists. Bishop Turrubius of Astorga, astonished at the spread of this sect in Spain, had addressed the other Spanish bishops on the subject, sending a copy of his letter to Leo, who did not let slip the opportunity to exercise influence in Spain. He wrote an extended treatise (July 21, 447) against the sect, examining its false teaching in detail, and calling for a Spanish general council to investigate whether it had any adherents in the episcopate -- but this was prevented by the political circumstances of Spain.
Leo enforced his authority in 445 against Dioscurus, Cyril's successor in the patriarchate of Alexandria, insisting that the ecclesiastical practise of his see should follow that of Rome; since Mark, the disciple of Peter and founder of the Alexandrian Church, could have had no other tradition than that of the prince of the apostles.
The fact that the African province of Mauretania Caesariensis had been preserved to the empire and thus to the Nicene faith in the Vandal invasion, and in its isolation was disposed to rest on outside support, gave Leo an opportunity to assert his authority there, which he did decisively in regard to a number of questions of discipline.
In a letter to the bishops of Campania, Picenum, and Tuscany (443) he required the observance of all his precepts and those of his predecessors; and he sharply rebuked the bishops of Sicily (447) for their deviation from the Roman custom as to the time of baptism, requiring them to send delegates to the Roman synod to learn the proper practise.
The assertion of Roman power over Illyria had been a strong point with previous popes. Pope Innocent I had constituted the metropolitan of Thessalonica his vicar, in order to oppose the growing power of the patriarch of Constantinople there. But now the Illyrian bishops showed a tendency to side with Constantinople, and the popes had difficulty in maintaining their authority. In 444 Leo laid down in a letter to them the principle that Peter had received the primacy and oversight of the whole Church as a requital of his faith, and that thus all important matters were to be referred to and decided by Rome. In 446 he had occasion twice to interfere in the affairs of Illyria, and in the same spirit spoke of the Roman pontiff as the apex of the hierarchy of bishops, metropolitans, and primates. However, after his death the influence of Constantinople was again predominant.