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I was asked this by email: I heard the statement on a video the other day that at the council of Nicea Mary was raised to the status of a goddess and made co-mediater with Christ. Is this true? Did the errors involved with belief about Mary creep in that early in Church history?
This is such an amalgamation (combination) of mistakes that it's funny.
The Council of Nicea did not address Mary. At the Council of Ephesus, one hundred years later (431), there was a debate about whether Mary could be called Theotokos, which means "mother of God." Nestorius, bishop of Constantinople,was against the use of the term. Mary, he taught, is the mother only of the body of Jesus. God has no mother.
He was opposed by Cyril, bishop of Alexandria. Cyril was for the use of Theotokos, Nestorius against.
It's pretty outrageous, but the supporters of each met separately rather than with their opponents. Both halves of the council excommunicated the leader of their opponents.
I have not researched the whole story because Christian-history.org focuses mostly on the pre-Nicene, Nicene, and post-Reformation period, but Cyril won in the end. Nestorius was excommunicated, and some of the churches of Assyria, who don't mind being referred to as Nestorian, remain excommunicated to this day.
The irony of this is that 19 years later, at the council of Chalcedon, the bishop of Alexandria was excommunicated over a similar issue having to do with the human and divine natures of Jesus. That schism remains unhealed as well, and the descendants of the excommunicated Alexandrian bishop are now known as the Coptic Orthodox.
The oldest reference to Mary as Theotokos dates to the mid-third century in a poem likely written in Alexandria. So maybe around AD 250 there were Alexandrian Christians calling Mary "mother of God."*
* I have heard repeatedly that a third-century papyrus from Egypt was the earliest reference to Mary as "mother of God." That may be true for the exact phrase, but Hippolytus, around AD 230 refers to "God-bearing Mary". I found that reference only after I answered the email above.
The "co-mediatrix" is only Roman Catholic. It's a much more recent idea.
The other thing you should know is that after so many Romans followed Constantine into the church, it became very corrupted. It became so corrupted that a lot of godly men left, becoming the hermits and monks of the fourth century. The emperors remained Christian until Julian the Apostate, who reigned from 360 to 363. Julian pushed for a return to paganism, but it was more an attempt at pagan evangelism than an imperial order. He regularly wrote in defense of paganism.
One of those writings by Emperor Julian accuses the Christians of more hero worship than the pagans practiced. (I have been unable to find the reference again, and any help would be appreciated!) I am pretty certain what happened is that all those people who moved from paganism to Christianity under the Christian emperors liked their pagan gods, so they just replaced those gods with Christian saints. The greatest of these in pagan eyes was Mary, of course, and she began growing in stature rapidly under the push to honor saints in the place of the pagan gods.
To give you a quick timeline, Constantine reigned until 337. Julian reigned from 360-363. Cyril and Nestorius fought over the use of "mother of God" at the Council of Ephesus in 432 (plus some years before the council).
It is in the Catholic Catechism (par. 969), but all I found on its origin was that Catholics are still waiting around to see if the pope will speak "ex cathedra," thereby infallibly (in their opinion) declaring Mary to be mediatrix. Even without that declaration, however, the fact that it is in the Catholic Catechism means that it is official RC doctrine.
You should look at my article on the myths of Nicea. It will tell you exactly what was covered at Nicea and how we know. It is a relatively short article with a lot of links.