Leo the Great, the bishop of Rome (i.e., the Pope) from AD 440-461, gave a series of sermons during his administration on the Feast of the Nativity (i.e., Christmas). The Christian church at the time was wrestling through the issue of understanding how Christ could be both fully man and fully God, producing what came to be known as the Definition of Chalcedon in AD 451. Leo's Christmas sermons were not simple pietistic homilies on the Baby Jesus in the manager. Instead, they combined deep theological reflection with rich devotion to teach people the mystery of Christ's human and divine natures.