Why is Mary called the Mother of God?

Mary is called the Mother of God because she is the mother of Jesus Christ, who is one divine Person — the eternal Son of God. Since the person she bore is truly God, the title 'Mother of God' (Theotokos, God-bearer) is correct and was solemnly defined by the Council of Ephesus in 431 AD.

Scriptural Basis

St. John opens his Gospel with the proclamation that 'the Word was God... and the Word was made flesh' (John 1:1, 14). The Word who became flesh in Mary's womb was, from eternity, God himself.