John Calvin, https://books.google.com/books?isbn=1556352468 Epistle CCC to the French church in London, 27th September 1552; translated by Jules Bonnet, p.362
“I do not doubt that there has been some ignorance in their having reproved this mode of speech, — that the Virgin Mary is the Mother of God … I cannot dissemble that it is found to be a bad practice ordinarily to adopt this title in speaking of this Virgin: and, for my part, I cannot consider such language as good, proper, or suitable… for to say, the Mother of God for the Virgin Mary, can only serve to harden the ignorant in their superstitions.”
Calvin to the Foreigners’ Church in London, 1552-10-27, in George Cornelius Gorham, Gleanings of a few scattered ears, during the period of Reformation in England and of the times immediately succeeding : A.D. 1533 to A.D. 1588 http://books.google.com/books?vid=0bbTMcT6wXFWRHGP&id=esICAAAAQAAJ&printsec=titlepage&dq=%22george+cornelius+gorham%22 (London: Bell and Daldy, 1857), p. 285.
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John Calvin 161
French Protestant reformer 1509–1564Related quotes
“I esteem immensely the Mother of God, the ever chaste, immaculate Virgin Mary.”
                                        
                                        Against the Galileans (c. 361) as translated in  The Works of the Emperor Julian,  http://books.google.com/books?id=ZGliAAAAMAAJ&q=%22But+why+do+you+not+cease+to+call+Mary+the+mother+of+God%22&dq=%22But+why+do+you+not+cease+to+call+Mary+the+mother+of+God%22&lr=&pgis=1 edited by Wilmer Cave Wright, London, W. Heinemann; New York, The Macmillan co., (1913 - 1923), volume 3, p. 399,  ISBN 0674990145 ISBN 9780674990142 . 
General sources
                                    
“It is an article of faith that Mary is Mother of the Lord and still a Virgin.”
Weimar edition of Martin Luther's Works, English translation edited by J. Pelikan [Concordia: St. Louis], Vol. 11, 319-320
“There can be no doubt that the Virgin Mary is in heaven. How it happened we do not know.”
Weimar edition of Martin Luther's Works (Translation by William J. Cole) Vol. 10, p. 268
Martin Luther as quoted in Tappert, Theodore G. (1959). The Book of Concord: the Confessions of the Evangelical Lutheran Church. Philadelphia: Fortress Press, p. 595
Zwingli Opera, Corpus Reformatorum, Volume 1, p. 424.
In Mary for Earth and Heaven: Essays on Mary and Ecumenism http://books.google.com/books?id=Dx4WrfzZMsoC&pg=PA116&dq=%22it+was+fitting+that+the+virgin+should+be+radiant+with+a+purity+so+great%22&hl=en&ei=ELNATrLoCIXMsQLbsvmuCQ&sa=X&oi=book_result&ct=result&resnum=1&sqi=2&ved=0CCoQ6AEwAA#v=onepage&q=%22it%20was%20fitting%20that%20the%20virgin%20should%20be%20radiant%20with%20a%20purity%20so%20great%22&f=false, 2002, William McLaughlin, Jill Pinnock, eds., Gracewing, ISBN 0852445563 ISBN 9780852445563pp. 115-116.
D. Martin Luthers Werke, Kritische Gesamtausgabe, 61 vols., (Weimar: Verlag Hermann Böhlaus Nochfolger, 1883-1983), 52:39 [hereinafter: WA] 1544