The French reformer John Calvin (1509-1564) also held that Mary was the Mother of God.
It cannot be denied that God in choosing and destining Mary to be the Mother of his Son, granted her the highest honor. ... 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. (Calvini Opera, Corpus Reformatorum, Braunschweig-Berlin, 1863-1900, v. 45, p. 348, 35.)
On the perpetual virginity of Mary, "Calvin
Routinely brushes aside the difficulties
sometimes raised from "first born" and
"brothers of the Lord."
(O'Carroll, M., 1983, Theotokos,
M Glazier, Inc.: Wilmington, DE, p. 94.)