At the beginning of the third session of the Second Vatican Council, Pope Paul VI announced that Mary would be honored under the title “Mother of the Church.”
From Christ’s conception until his death, Mary was united to her Son in his work of salvation. From the Cross, Jesus entrusted his beloved disciple to Mary, telling him to see her as his own mother (Jn 19:27). When the Apostles and disciples gathered to pray after the Ascension of Jesus, Mary was with them praying for the coming of the Holy Spirit. Mary continues to pray before God for the Church and all humanity.
Like Mary, the Church has a maternal role, giving birth to people in Christ. The Church can never cease to look at Mary, who gave birth to Jesus Christ. The Church contemplates Mary’s motherhood in order to fulfill her own calling to be mother of the members of Christ’s Mystical Body, the Church. Also like Mary, the Church is virginal. The description of the Church as virginal is used here in the spiritual sense of the undivided heart and of fidelity in its most luminous form. God calls all the members of the Church to fidelity to the union with him begun at Baptism and continued in the other Sacraments.
In our culture, there can be a discomfort with praying for Mary’s intercession on our behalf. This seems to be a mediating role that crosses a line set out in the First Letter to Timothy: “For there is one God. / There is also one mediator between God and the human race, / Christ Jesus, himself human / who gave himself as a ransom for all” (1 Tm 2:5). So Jesus Christ is the one and only mediator. Jesus alone is the Savior.
But this does not deny the possibility that Christ would permit others to share in his mediating role. Here on earth we routinely ask others for prayers. Instinctively, we turn to holy people for their prayers because they seem nearer to God. Why would we stop asking saints for their prayers after they die? If we believe they are in heaven, would not their prayers be even more effective?
From the earliest times, Christians have sought Mary’s prayers and help. There has been the basic sense on the part of the Church that Mary continues in heaven to be concerned for the growth of all members of the Church into holiness and an intimate relationship with her Son.
You can read more from the United States Catholic Catechism for Adults, order your own copy, or read questions about it at the United States Conference of Catholic Bishops website.
If someone ask you why Catholics call Mary the mother of God, here are but a few answered you can give based on the Scriptures.
John 1:1..in the beginning the word(Jesus) already existed,
The word (Jesus) was with God and the word(Jesus) was God.
From the very beginning, the word(Jesus)was with God, through him, God made all things, not one thing in all creation was made without him.
The word(Jesus) was the source of life and this life brought life to humanity….John1-1:4
The word(Jesus) was in the world and though God made the world through him, the world did not recognize him.
John 1:14—the word(Jesus ) became a human being and full of grace and truth, dwelt among us.
The bible also made it known to us in john.1:18″no one has ever seen God ,
The only son ,who is the same as God and is at the fathers side has made him known
Jesus is the only son of God and also the God we see…
This is only but a simple layman’s answer to why we call our Mother Mary “Mother of God.”