The Libri Carolini, Opus Caroli regis contra synodum, also called Charlemagne's Books or simply the Carolines, are the work in four books composed on the command of Charlemagne, in the mid 790s, to refute the conclusions of the Byzantine Second Council of Nicaea (787), particularly as regards its acts and decrees in the matter of sacred images. They are "much the fullest statement of the Western attitude to representational art that has been left to us by the Middle Ages". Two earlier Frankish tracts against images had been sent in 792 to Pope Hadrian I, who had replied with an attempt at a refutation. The Libri Carolini was then composed as a fuller rebuttal of Hadrian's position. But Charlemagne realized that further controversy with Rome would serve no purpose, and the work was never sent. It remained unknown until it was published in 1547, in the very different context of the debates over images at the Reformation. The work in no way recommends the destruction of images but deplores a cult of images; it thereby anticipated the Lutheran rather than the Calvinist attitude to religious art. Despite this John Calvin refers to it approvingly in later editions of his Institutes of the Christian Religion, and uses it in his argument against the veneration of images.