T. H. L. Parker theorizes that, having become learned in both Hebrew and Greek, he preached directly from his Hebrew Old Testament and his Greek New Testament. In some sermons and series of sermons, the French text recorded does not match any known French translation. In addition, when repeating the text later in the sermon or in another sermon, his text does not match what was said before. This would seem to indicate that Calvin did not preach from a French translation.
Considering the poor view that Calvin is known to have had of the Vulgate, one may also reasonably assume that he did not use a Latin Bible. All of this has led Parker to follow the line of reasoning put forth by the editors of Calvin’s sermons on Isaiah 30-41 that he preached directly from his Hebrew Old Testament, translating as he went along. From this, Parker also deduces that Calvin preached directly from his Greek text when preaching his sermons on the New Testament.
I would therefore put forward the hypothesis that Calvin took a Hebrew or Greek text into the pulpit and translated as he went along, just as he did for his lectures. The indications to the contrary arise from inconsistency in the transmission of the sermons.
Calvin was then very familiar with the Scriptures in their original languages and thus studied, lectured and preached from them.
One may also surmise that since he lectured without notes it is highly likely that he preached without notes as well. Calvin was renowned for his excellent memory and therefore he probably trusted his memory to aid him in his preaching.
The absence of notes, however, does not presuppose that he preached without preparation. Indeed Calvin himself, in his sermon on Deuteronomy 6:16 makes it plain that only arrogance and pride would lead one into the pulpit thinking preparation an unnecessary part of preaching..
If I should climb up into the pulpit without having deigned to look at a book and frivolously imagine, “Ah, well! When I get there God will give me enough to talk about,” and I do not condescend to read, or to think about what I ought to declare, and I come here without carefully pondering how I must apply the Holy Scripture to the edification of the people – well, then I should be a cock-sure charlatan and God would put me to confusion in my audaciousness.
So study was an important part of the pastoral work to John Calvin. He studied the Scriptures to find the meaning of the text, the meaning the original writer intended to convey and the meaning the text held by way of application for the congregation that would hear his exposition of it.
 T. H. L. Parker, Calvin’s Preaching, (Louisville: Westminster/John Knox Press, 1992), 80-81. See also Appendix 3 in this same volume for a more thorough discussion of and proofs for Parker’s reasoning.
 Ibid., 177.
 As cited in Ibid., 81.