One of the most basic questions we can ask about God is how, exactly, are we to worship Him? Scripture is certainly not silent on this topic. Many acts of worship are described, such as Baptism and prayer. This is simple enough, but obviously that's not the end. The Bible tells us we must worship God in spirit and in truth (John 4:24). How, exactly, do we do this? Two competing theories exist.
The first is the normative principle of worship. This principle effectively states that anything not explicitly forbidden in Scripture is implicitly allowed. One could make a rather strong case for this, and where Christian liberty is concerned, I'd say the case is nigh unassailable. It does, however, run the risk of inviting clearly unbiblical beliefs and practices into the Church. For example, the Bible does not explicitly forbid calling God "Mother Nature", but unless you're sorely lacking in Biblical knowledge, this ought to make you cringe.
Enter the Regulative principle, a tighter position that not only conforms to the doctrine of Sola Scriptura, but also hangs on the premise that God is wiser than we are, particularly with regard to Himself, and thus has the sole right to dictate how He is to be worshiped. We, on the other hand, tend to goof even when it comes to the commands He has given us, so how could we be rightly expected to come up with our own rules?
The Regulative principle finds a lot of support in both Old and New Testaments. For example, in Deuteronomy 4:2, we read "Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you." Similarly, in Deuteronomy 12:32, we read "What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it." Here, not only do we have two very explicit commands, but we also have a very clear reason for it; you don't add or take from it in order to ensure obedience.
This is especially visible in the New Testament, where Jesus frequently clashes with the Pharisees. These men often "...made the commandment of God of none effect..." by their traditions (Matthew 15:1-6). They "...strain at a gnat, and swallow a camel." (Matthew 23:24). They constantly ignored the weightier matters of the law (Matthew 23:23; Luke 11:42). Basically, these were impious men with the illusion of piety, all because they added to, and took from, the word of God.
Further evidence for the Regulative principle comes from the so-called "book of wisdom", i.e. Proverbs. In chapter 30, verses 5 and 6, we read "Every word of God is pure: he is a shield unto them that put their trust in him. Add thou not unto his words, lest he reprove thee, and thou be found a liar." Here we see the reiteration that adding to God's words is not wise, to say the least, not only reasoning that this is because the word of God is both pure and a shield to those who trust Him, but also because those who are foolish enough to add to them will be rebuked as liars.
Some might say this is just Old Testament stuff. This is a very strange objection, even under the Normative principle. While we are certainly no longer under the law, we are nevertheless forbidden to diminish it. It remains the word of God, and Jesus said those who break the least of the commandments of the law shall be called least in the Kingdom of Heaven (Matthew 5:19). In other words, the Old Testament may have a different modern application, but to entirely dismiss it is troublesome, to say the least.
Furthermore, while we are no longer under the law, it continues to give us principles, a lot of which are actually reiterated in the New Testament. For example, of the 10 commandments, only the observance of the Sabbath is not renewed in the New Testament. Commands against idolatry, blasphemy, theft, murder, covetousness etc. are all still in force. Even the command to observe the Sabbath has effectively "ascended", rather than been repealed, due to its fulfilment in Christ. He is now our Sabbath rest, and so we observe the Sabbath simply by relying on Him for our salvation.
Just as we can rely on the Old Testament to give us principles regarding the sanctity of life, property rights, and Monotheism, we can rely on it to give us principles of worship. In fact, much like the other commandments, the Regulative principle is just as strongly supported by the New Testament, if not more so. For example, in Colossians 2:13-23, we read "And this I say, lest any man should beguile you with enticing words. For though I be absent in the flesh, yet am I with you in the spirit, joying and beholding your order, and the stedfastness of your faith in Christ. As ye have therefore received Christ Jesus the Lord, so walk ye in him: Rooted and built up in him, and stablished in the faith, as ye have been taught, abounding therein with thanksgiving. Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ. For in him dwelleth all the fulness of the Godhead bodily. And ye are complete in him, which is the head of all principality and power: In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead. And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses; Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it. Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ. Let no man beguile you of your reward in a voluntary humility and worshipping of angels, intruding into those things which he hath not seen, vainly puffed up by his fleshly mind, And not holding the Head, from which all the body by joints and bands having nourishment ministered, and knit together, increaseth with the increase of God. Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances, (Touch not; taste not; handle not; Which all are to perish with the using;) after the commandments and doctrines of men? Which things have indeed a shew of wisdom in will worship, and humility, and neglecting of the body: not in any honour to the satisfying of the flesh."
This passage not only presents the Regulative principle, but also disarms a common objection to it. First of all, note how, especially in verse 8, it rebukes the notion of human tradition holding any authority what so ever. "Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ." Well there we go. Christ, not the tradition of men, is our standard.
Notice how the passage also deals with both dos and do nots. Do worship angels. Do not touch, taste, or handle. These additions to the word of God go both ways. While the worship of angels is, of course, forbidden, the other things are not, yet the Colossians apparently needed to be warned against the addition of tradition.
Yet, at the same time, Paul actually shows that the Regulative principle, while it certainly exists, does not have to prohibit liberty. Proponents of the Normative principle like to point to things that aren't prescribed by Scripture that are still practiced by proponents of the Regulative principle. For example, Scripture does not say we must have church buildings, and it certainly doesn't prescribe things like microphones. It doesn't talk about a lot of things we practice in Christianity today. But this argument fails to understand what the Regulative principle is.
The Regulative principle is not the hard line stance that anything not explicitly prescribed by Scripture is utterly forbidden, no exceptions. Rather, it is the principle that God's word is sufficient (2 Timothy 3:14-17), giving us commands, but not always being specific on how to carry them out. The Bible tells us not to neglect to meet together, but it doesn't say "hey, go meet at this house, don't build a building". So we can build buildings. It tells us to sing new songs, and to worship with hymns and Psalms, but it doesn't give us lyrics, nor a specific set of instruments (in fact, it gives us a rather open-ended list) that can be used. Of course, issues of Christian liberty, such as whether or not to celebrate Christmas, are quite explicitly left to the conscience of the individual, and so the Regulative principle, at least to an extent, actually leads in to the Normative principle, but with safeguards against the traditions that are liable to mess us up.
One could think of it this way: Imagine if Jesus told you "eat this banana". There are more than one way to open a banana. One way is to simply grab the stalk, which previously attached it to the rest of the bunch, and pull the skin down, but another way, which I'm told is the way chimps do it in nature, is to pinch the bottom, splitting it in half, and then using that to remove the skin. The end result is the same: The skin is removed from the banana, and it is now completely edible. Christ told you to eat it, so nom nom. It doesn't matter how you peel it. In the same way, as long as Biblical commands are fulfilled, the how does not matter (unless, of course, the how is also specified). There you have the Regulative principle.
But the Normative principle leaves it open ended even beyond the fulfilment of the command. Christ told me to eat this banana, He didn't say eat this orange. I therefore conclude He will be pleased if I eat this orange. What evidence do you have that He will be pleased if you eat the orange? None what so ever. You just happened to believe eating the orange was pleasing to God. This is when worship just gets silly. You have the appearance of piety, but in reality, you're just adding a bunch of things to God's word, and it's only a matter of time before He rebukes you for it.
And so we see that the Regulative principle of worship, owing to its deference to God in His wisdom, by obedience to His inspired word, is vastly superior to the Normative principle, which takes man's role in worship from follower to leader. Where Scripture does give us permission to go our own way, there is obviously room to do so, but ordinarily, the Biblical position seems to be that what God tells us He wants, He wants, and we must obey, but what He doesn't tell us He wants is, in all likelihood, something He would prefer we avoid, and at the very least won't make Him happy. Unless we want to live in an era in which smearing ourselves with strawberry jam and singing "I've got a lovely bunch of coconuts" is considered valid worship, the Regulative principle should be considered correct and adhered to.