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.[13]

In speaking of various corruptions of worship, Calvin comments:

I come now to ceremonies, which, while they ought to be grave attestations of divine worship, are rather a mere mockery of God. A new Judaism, as a substitute for that which God has distinctly abrogated, has again been reared up by means of numerous puerile extravagancies, collected from different quarters; and with these have been mixed up certain impious rites, partly borrowed from the heathen, and more adapted to some theatrical show than to the dignity of our religion. The first evil here is, that an immense number of ceremonies, which God had by his authority abrogated, once for all, have been again revived. The next evil is, that while ceremonies ought to be living exercises of piety, men are vainly occupied with numbers of them that are both frivolous and useless. But by far the most deadly evil of all is, that after men have thus mocked God with ceremonies of one kind or other, they think they have fulfilled their duty as admirably as if these ceremonies included in the whole essence of piety and divine worship.[14]

And in yet more pointed remarks, Calvin says:

The mockery which worships God with nought but external gestures and absurd human fictions, how could we, without sin, allow to pass unrebuked? We know how much he hates hypocrisy, and yet in that fictitious worship, which was everywhere in use, hypocrisy reigned. We hear how bitter the terms in which the prophets inveigh against all worship fabricated by human rashness. But a good intention, i.e., an insane license of daring whatever man pleased, was deemed the perfection of worship. For it is certain that in the whole body of worship which had been established, there was scarcely a single observance which had an authoritative sanction from the Word of God.

We are not in this matter to stand either by our own or by other men’s judgments. We must listen to the voice of God, and hear in what estimation he holds that profanation of worship which is displayed when men, overleaping the boundaries of His Word, run riot in their own inventions. The reasons which he assigns for punishing the Israelites with blindness, after they had lost the pious and holy discipline of the Church, are two, viz., the prevalence of hypocrisy, and will-worship (ejqeloqrhskeiva), meaning thereby a form of worship contrived by man. “Forasmuch,” says he, “as the people draw near me with their mouth, and with the lips do honor me, but have removed their heart far from me, and their fear toward me is taught by the precept of men; therefore I will proceed to do a marvellous work among this people, even a marvellous work and a wonder: for the wisdom of their wise men shall perish, and the understanding of their prudent men shall be hid,” Isa. 29:13-14. When God stirred us up, a similar or worse perversity openly domineered throughout the Church. While God, then, was thundering from heaven, were we to sit quiet?[15]

Thus, Calvin’s writings on worship clearly enunciate the concept which has subsequently been called the regulative principle of worship: all modes of worship must be expressly sanctioned by God’s word, if they are to be considered legitimate. Since Christmas observances, and other ecclesiastical festivals, are not commanded in the scriptures, they fail to meet divine approval, even if there were no additional objections to them.

Source: http://www.swrb.com/newslett/actualNLs/Xmas_ch2.htm

God forbid: yea, let God be true, but every man a liar; as it is written, That you might be justified in your sayings, and might overcome when you are judged