The great Bishop of Constantinople, St. John Chrysostom (d. 407), composed a Eucharistic Prayer for the Divine Liturgy (Holy Mass) still in use today:

“It is truly just to proclaim you blessed, O Mother of God, who are most blessed, all pure and Mother of our God. We magnify you who are more honorable than the Cherubim and incomparably more glorious than the Seraphim. You who, without losing your virginity, gave birth to the Word
of God. You who are truly the Mother of God.”

In June 431, the Council of Ephesus was convened. That great Council insisted “If anyone does not confess that the Emmanuel is truly God and therefore that the holy Virgin is the Mother of God “Theotokos” (since she begot according to the flesh the Word of God made flesh), anathema sit.” The Council affirmed that Jesus Christ is One Person, with two natures–human and divine and that this is a union which cannot be separated.

The Council’s affirmation that Mary can rightfully be called the Mother of God is thus profoundly “Christological”, affirming who Jesus is and who we can become in Him. Mary is not Mother of God, the Father, or Mother of God, the Holy Spirit; rather, she is Mother of God, the Son – Jesus Christ.

The titles given to Mary have never been viewed as in any way diminishing the unique role of her Son and Savior Jesus Christ. They were the fruit of men and women who, inspired by the Spirit, were given profound insights into the meaning of Jesus Christ and the work of salvation (catholic.org, Catholic Online, 2019).

January 1st, the Solemnity of Mary Mother of God, is a Holy Day of Obligation in the Roman Catholic Church. On this day (or the preceding evening), Catholics are obliged to attend Mass honouring the Blessed Virgin Mary as Mother of the Son of God.