I know how difficult it is to persuade the world that God disapproves of all modes of worship not expressly sanctioned by His Word. The opposite persuasion which cleaves to them, being seated, as it were, in their very bones and marrow, is, that whatever they do has in itself a sufficient sanction, provided it exhibits some kind of zeal for the honor of God. But since God not only regards as frivolous, but also plainly abominates, whatever we undertake from zeal to His worship, if at variance with His command, what do we gain by a contrary course? The words of God are clear and distinct, “Obedience is better than sacrifice.” “In vain do they worship me, teaching for doctrines the commandments of men,” 1 Sam. 15:22; Matt. 15:9. Every addition of His word, especially in this matter, is a lie. Mere “will worship” (ethelothreeskia) is vanity [Col. 2:23]. This is the decision, and when once the judge has decided, it is no longer time to debate.
Source: Calvin, Tracts (1844; rpt. Grand Rapids: Baker, 1983), Vol. 1, pp. 128-29, http://www.swrb.com/newslett/actualNLs/Xmas_ch2.htm