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.”
Thanks