I wanted to share J.N.D. Kelly’s summarizing of the theses presented in Pope Leo I’s Tome. The writings which helped contribute to what became known as the Chalcedonian settlement which occurred at the Council of Chalcedon in 451ad. It is this “settlement” which has been used, thenceforth, as the standard or canon for determining whether or not someone’s view of Jesus Christ is orthodox or heterodox, if not downright heretical. As you will see through Kelly’s summary what Leo offered in his Tome wasn’t necessarily original to him, instead it served as a good codification of what had come before him in the various christological struggles (which the Council of Nicaea in 325ad is related to in some important conceptual matters). Here is Kelly:
The Christology which appears in Leo’s Tome has no special originality; it reflects and codifies with masterly precision the ideas of his predecessors. The following are the chief points he was concerned to bring out. First, the Person of the God-man is identical with that of the divine Word. As he expressed it, ‘He Who became man in the form of a servant is He Who in the form of God created man’. Though describing the incarnation as ‘self-emptying’ (exinanitio), he claimed that it involved no diminution of the Word’s omnipotence; He descended from His throne in heaven, but did not surrender His Father’s glory. Secondly, the divine and human natures co-exist in this one Person without mixture or confusion. Rather, in uniting to form one Person each retains its natural properties unimpaired (salva . . . proprietate utriusque naturae et substantiae), so that, just as the form of God does not do away with the form of a servant, so the form of a servant does not diminish the form of God. Indeed, the redemption required that ‘one and the same mediator between God and men, the man Jesus Christ, should be able to both die in respect of the one and not to die in respect of the other’. Thirdly, the natures are separate principles of operation, although they always act in concert with each other. So we have the famous sentence, ‘Each form accomplishes in concert with the other what is appropriate to it, the Word performing what belongs to the Word, and the flesh carrying out what belongs to the flesh’. Lastly, the oneness of the Person postulates the legitimacy of the ‘communication of idioms’. We can affirm, for example, that the Son of God was crucified and buried, and also that the Son of Man came down from heaven.
These four theses may not have probed the Christological problem very deeply; it is obvious that they left the issues which puzzled Greek theologians largely untouched. They had the merit, however, of setting out the factors demanding recognition fairly and squarely. Moreover, they went a long way towards meeting the points of view of both the schools of thought struggling for supremacy in the East. Antiochenes could recognize their own theology in Leo’s vigorous affirmation of the duality in Christ, and of the reality and independence of the two natures. Some of his sentences, indeed, particularly the one cited above, were to prove stones of stumbling to Alexandrian Christologians. Nevertheless these latter, too, could see the essentials of their standpoint vindicated in the Pope’s unerring grasp of the identity of the Person of the Incarnate with that of the eternal Word. As he expressed it in a Christmas sermon, ‘It is one and the same Son of God Who exists in both natures, taking what is ours to Himself without losing what is His own’.
It may or may not trouble some that Leo was a Roman Pope, but what this should illustrate for Christians across the spectrum is that we share an ecumenical past when it comes to the most basic stuff of our theological grammar and how we understand who God has revealed Himself to be in His Son, Jesus Christ. Beyond that, it is important to recognize that what we take for granted today as orthodoxy, when we speak of Christ’s two natures and the hypostatic union, or the Trinity, was something that developed over time within the mind of the church. We can be the most Free non-denominational Bible church out there, but it is important to remember that the orthodoxy we affirm when it comes to two-nature Christology, etc. is something that binds us to the church catholic itself. It is these realities, and church historical developments that ought to cause people who claim a nuda scriptura or solo Scriptura approach (meaning people who often claim the label of Biblicist) to come to terms with the fact that even they operate with some very basic tradition as the foundation for how they conceptualize God and Jesus Christ; which of course then impacts the way they interpret and read Holy Scripture itself.
 J.N.D. Kelly, Early Christian Doctrines. Revised Edition (San Francisco: HarperSanFrancisco, 1978), 337-38.