In the first place Tradition?

"Our first plea and entreaty was that he [(Sébastien Châteillon/Sebastian Castellio)] should not rashly reject the age-long interpretation of the whole of the Church."

"Principio obtestati eum sumus, ut ne perpetuum universae ecclesiae consensum temere pro nihilo duceret."

Above all we begged him not rashly to consider as nothing the unbroken consensus of the universal church.

     John Calvin for the ministers of the church of Geneva, Letter no. 531 of early 1544 recommending Sébastien Châteillon/Sebastian Castellio (who had been denied admission to the Genevan company of pastors) for another (teaching?) post in Lausanne, as trans. G. R. Potter and M. Greengrass, eds., John Calvin, Documents of modern history (London:  1983), 101, as quoted in Diarmaid MacCulloch, The Reformation:  a history (New York:  Viking, 2003), 235 =705n34.  This appears to me to be CO 11, col. 675 (674-676), above (translation mine).  Cf. Wulfert de Greef, The writings of John Calvin:  an introductory guide, trans. Lyle D. Bierma, expanded ed. (Louisville:  Westminster John Knox, 2008), 33-34.  At issue was "the unbroken consensus of the universal church" on the canonicity of the Song of Songs, and (for MacCulloch) what Trent would in a few years make of that.  Note, however, that I have not yet read this in the context of the letter or Calvin's theology as a whole!