There was a heretical bishop in Constantinople in the first part of the 5th century who insisted that Jesus could not be a single subject, a single Person with two natures, divine and human. Against this the Council of Ephesus in a.d. 431 confessed the unity of Christ's Person, and that, as a strong Christological statement, that Mary is the Mother of God. So must every Christian confess, for we must be confident of who our Lord Jesus is. He is, as Thomas rightly put it, my Lord and my God. St. Paul's on the Air for June 2, 2019.
This Council was held in Constantinople, in ad 381, just over half a century after the first. It expanded the Creed into its current form, reaffirming the deity of Christ and confessing the Person of the Holy Ghost. Here's the podcast.