30 As soon as the watchful Prelate perceived that the errors of Nestorius were spreading and growing, with increasing danger to the orthodox faith, he wrote to Our Predecessor St. Celestine I in the following terms: Since God requires us to be vigilant in these matters, and since the ancient custom of the Church persuades us that questions of this kind should be communicated to Your Holiness, I write, driven by necessity.
Source: Orientalis Ecclesiae (Vatican.va)