St. Sixtus III, 432-440

["The formula of union" of the year 433, by which the peace between St. Cyril of Alexandria and the Antiochenes was established, was approved by St. Sixtus III]

But how we know and speak regarding the Virgin Mother of God, and about the manner of the incarnation of the only-begotten Son of God, necessary not because of increase but for satisfaction, we have taken and possess from above, from the divine Scriptures as well as from the tradition of the holy fathers, and we speak briefly, adding nothing at all to the faith of the holy Fathers, which was set forth at Nicea. For, as we have already said, this suffices for all understanding of piety and for all renunciation of heretical perfidy. But we speak not presuming the unlawful, but by confession of special weakness excluding those who wish to rise up against what we regard as beyond man.

We confess our Lord Jesus Christ, the only begotten son of God, perfect God and perfect man, of a rational soul and of a body, born of the Father before the ages according to the Godhead, but in the last days the same on account of us and on account of our salvation according to the incarnation from the Virgin Mary, consubstantial with the Father, the same according to the Godhead, and consubstantial with us according to the incarnation. For the unity of the two natures was made; wherefore, we confess one Christ, one son, one Lord. According to this unmingled unity we confess the holy Virgin Mother of God, because the Word of God was made flesh and was made man, and by the conception united to Himself a temple assumed from her. Moreover, we recognize the evangelical and apostolic voices about the Lord as men speaking with divine inspiration, joining these sometimes as if spoken of one person, but sometimes separating them as if of two natures, and these indeed befitting God according to the Godhead of Christ, but humbly teaching according to the incarnation.