Over the past several decades Leonine studies have focused attention on Pope Leo the Great's Christology, noting the influence his Tomus ad Flavianum had at the Council of Chalcedon. In fact, because of this strong influence twentieth-century scholars have studied the Tome nearly exclusively in order to identify the heart of Leo's Christology. There can be no question, of course, that the Tome should be consulted in order to understand Leo's Christology, but it marks only one phase in the ongoing development of the ways he chose to express his christological insights. In part the Tome itself precipitated this development insofar as it opened up his Christology to scrutiny in the East. The tone of Leo's insights and the language he used to express them shifted and acquired greater precision over time in his letters and sermons in direct response to the dynamics of the christological controversy in the East, of which Leo's Tome made him a part. This development is most evident in three areas: his avoidance of the “Mother of God” title for the Virgin Mary after initially using it early in his pontificate; his use of the terms homo and humanus, which Leo learned to distinguish later in his pontificate; and his adoption of the Antiochene homo assumptus formula late in his pontificate to emphasize the fullness of Christ's human nature. These phenomena reflect the pope's careful attempt to distance himself from the rising tide of the Monophysite movement in the East, as he began to channel his traditional, Western Christology more through formulae used by Antiochene theologians. These phenomena can only be observed through careful, chronological analysis of the broader corpus of Pope Leo's works.