It appears from the following Luther and Calvin quotes that they both initially believed that the Catholic Church is the true Church of Christ.
“Accordingly, we concede to the papacy that they sit in the true Church, possessing the office instituted by Christ and inherited from the apostles, to teach, baptize, administer the sacrament, absolve, ordain, etc., just as the Jews sat in their synagogues or assemblies and were the regularly established priesthood and authority of the Church. We admit all this and do not attack the office, although they are not willing to admit as much for us; yea, we confess that we have received these things from them, even as Christ by birth descended from the Jews and the apostles obtained the Scriptures from them.”
Sermon for the Sunday after Christ’s Ascension; John 15:26-16:4 (2nd sermon) A Sermon by Martin Luther; taken from his Church Postil, 1522. [This sermon is taken from volume III:254-271 of The Sermons of Martin Luther]
“But let us forgive them this, and let them take for granted that primacy was divinely bestowed on the Romish see, and has been sanctioned by the uniform consent of the ancient church; still there is room for this primacy only on the supposition that Rome has both a true church and a true bishop”
“For several centuries that see (the Roman See) has been possessed by impious superstitions, open idolatry, [and] perverse doctrines, while those great truths, in which the Christian religion chiefly consists, have been suppressed”
John Calvin – The Necessity of Reforming the Church (1543)
(taken from: http://www.protestanterrors.com/truechurch.htm )