THE THEOLOGIAN
Philip Melanchthon, Luther's close friend and colleague and himself no mean theologian, reportedly dubbed Calvin “the theologian.” Other Reformation leaders, including Luther, published theological treatises of significance. On the Reformed side, Zwingli, Bullinger, Bucer, Beza, and Peter Martyr made theological contributions. Yet it was only Melanchthon who wrote something like a systematic theology with his Loci Communes (1521 and later editions). Even so, Melanchthon was quite willing to acknowledge that Calvin was without peer when it came to theology.
The Institutes
That reputation originally resulted from the favorable reception of
Calvin’s magnum opus, the Institutes of the Christian Religion, which has
been hailed as one of the books “that has changed the world.” A more
modest assessment, and one generally recognized, is that it is a classic of
Protestant theology. It should be kept in mind, however, that the first edition
of the Institutes (1536) takes up only 243 pages in volume I of the opera
selecta, whereas the second edition (1539), in which Calvin really comes
into his own, is three times as large; and the final edition of 1559 is almost
five times as large as the first edition.