The early "reformers" believed that Mary was the mother of God.
Martin Luther
“She is rightly called not only the mother of the man, but also the Mother of God ... It is certain that Mary is the Mother of the real and true God." (Pelikan, J (ed), The Works of Martin Luther, Concordia: St. Louis, volume 24, 107.)
John Calvin
"Elizabeth called Mary Mother of the Lord, because the unity of the person in the two natures of Christ was such that she could have said that the mortal man engendered in the womb of Mary was at the same time the eternal God.” (Calvin, J, Opera, Braunshweig-Berlin, 1863-1900, Volume 45, 35.)
Ulrich Zwingli
“I esteem immensely the Mother of God, the ever chaste, immaculate Virgin Mary.” (Stakemeier, E, in De Mariologia et Oecumenismo, Balic, K (ed), Rome, 1962, p.456.)
It was only later as Protestatism splintered into myriad denominations, sects ands cults that they began to deny the truhs about Mary.