I have also no difficulty in conceding to you, that there is nothing more perilous to our salvation than a preposterous and perverse worship of God. The primary rudiments, by which we are wont to train to piety those whom we wish to gain as disciples to Christ, are these; viz., not to frame any new worship of God for themselves at random, and after their own pleasure, but to know that the only legitimate worship is that which he himself approved from the beginning. For we maintain, what the sacred oracle declared, that obedience is more excellent than any sacrifice (1 Sam. xv. 22.) In short, we train them, by every means, to be contented with the one rule of worship which they have received from his mouth, and bid adieu to all fictitious worship.
John Calvin, Reply to Cardinal Sadolet’s Letter (1539) in Tracts and Letters relating to the Reformation, trans and ed. Henry Beveridge, (7 vols, Edinburgh: Calvin Translation Society, 1844), 1: 34.