Before burning me at the stake, please remember that it was Pope Francis who first referred to this verse!
The phrase doesn’t have to be in Acts 1:14. Catholics believe that Jesus was God, and Mary was his mother.
A woman is a man’s mother either if she carried him in her womb or if she was the woman contributing half of his genetic matter or both. Mary was the mother of Jesus in both of these senses; because she not only carried Jesus in her womb but also supplied all of the genetic matter for his human body, since it was through her—not Joseph—that Jesus "was descended from David according to the flesh" (Rom. 1:3).
Since Mary is Jesus’ mother, it must be concluded that she is also the Mother of God: If Mary is the mother of Jesus, and if Jesus is God, then Mary is the Mother of God.
Although Mary is the Mother of God, she is not his mother in the sense that she is older than God or the source of her Son’s divinity, for she is neither. Rather, we say that she is the Mother of God in the sense that she carried in her womb a divine person—Jesus Christ, God "in the flesh" (2 John 7, cf. John 1:14)—and in the sense that she contributed the genetic matter to the human form God took in Jesus Christ.
“...Nowhere do I read the mother of God...”
If Jesus was God, and Mary was his mother, then Mary was the mother of God. Quite simple, really, unless one doesn’t believe that Jesus was Divine.
Really? We’re going to have yet another thread about whether Mary is the Mother of God?
Please read the Visitiation again.
Elizabeth greets Mary: “And how is this that the mother of my Lord should come to me?”
Then she goes on to talk about John the Baptist in her womb, leaping for joy.
These things are in the Bible for a reason.
My soul magnifies the Lord,
And my spirit rejoices in God my Savior.
For He has regarded the low estate of His handmaiden,
For behold, henceforth all generations shall call me blessed.
For He who is mighty has done great things for me, and holy is His name. And His mercy is on those who fear Him from generation to generation.
He has shown strength with His arm:
He has scattered the proud in the imagination of their hearts.
He has put down the mighty from their thrones,
and exalted those of low degree.
He has filled the hungry with good things;
and the rich He has sent empty away.
He has helped His servant Israel, in remembrance of His mercy;
As He spoke to our fathers, to Abraham and to His posterity forever.
Glory be to the Father and to the Son and to the Holy Spirit.
As it was in the beginning, is now and ever shall be, world without end. Amen
Magníficat ánima mea Dóminum,
et exsultávit spíritus meus
in Deo salvatóre meo,
quia respéxit humilitátem
ancíllæ suæ.
Ecce enim ex hoc beátam
me dicent omnes generatiónes,
quia fecit mihi magna,
qui potens est,
et sanctum nomen eius,
et misericórdia eius in progénies
et progénies timéntibus eum.
Fecit poténtiam in bráchio suo,
dispérsit supérbos mente cordis sui;
depósuit poténtes de sede
et exaltávit húmiles.
Esuriéntes implévit bonis
et dívites dimísit inánes.
Suscépit Ísrael púerum suum,
recordátus misericórdiæ,
sicut locútus est ad patres nostros,
Ábraham et sémini eius in sæcula.
Glória Patri et Fílio
et Spirítui Sancto.
Sicut erat in princípio,
et nunc et semper,
et in sæcula sæculórum.
Amen.
She became the Mother of God, in which work so many and such great good things are bestowed on her as pass man’s understanding. For on this there follows all honor, all blessedness, and her unique place in the whole of mankind, among which she has no equal, namely, that she had a child by the Father in heaven, and such a Child . . . Hence men have crowded all her glory into a single word, calling her the Mother of God . . . None can say of her nor announce to her greater things, even though he had as many tongues as the earth possesses flowers and blades of grass: the sky, stars; and the sea, grains of sand. It needs to be pondered in the heart what it means to be the Mother of God.
(Commentary on the Magnificat, 1521; in Luther’s Works, Pelikan et al, vol. 21, 326)