Theotokos means ‘God-bearer’ and is conventionally rendered ‘Mother of God.’ It does not claim that Mary created the Godhead. It confesses that the person conceived and born from her is the eternal Word truly made flesh. Fourth-century writers use the title before the Council of Ephesus, and Gregory of Nazianzus treats refusal to confess Mary as Theotokos as a Christological error.
At Ephesus in 431, the issue was whether the divine Word and the man Jesus could be spoken of as if they were two personal subjects. The council followed Cyril of Alexandria in confessing one Son and Lord. Mary’s title therefore serves the mystery of the Incarnation: the one she bore is not merely a Christ-bearing man but God the Word in his own humanity.
Primary source
Council of Ephesus, acts and Cyril’s letter to Nestorius: https://www.newadvent.org/fathers/3810.htm
Source notes
Primary source: the Council of Ephesus (431), its acts and St. Cyril’s letter to Nestorius, New Advent.