And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the Lord, which he commanded them not. And there went out fire from the Lord, and devoured them, and they died before the Lord. Then Moses said unto Aaron, This is it that the Lord spake, saying, I will be sanctified in them that come nigh me, and before all the people I will be glorified. And Aaron held his peace. (Leviticus 10:1-3)

Jeremiah Burroughs, Gospel Worship, pp. 3, 8-9.

What was their sin?

Their sin it was offering of strange fire, so the text saith that they offered strange fire, which God commanded them not, in the beginning of the chapter. But had God ever forbidden it? Where do we find that ever God had forbidden them to offer strange fire, or appointed that they should offer only one kind of fire? There is no text of Scripture that you can find from the beginning of Genesis to this place, where God hath said in terminis, in so many words expressly, ‘You shall offer no fire but one kind of fire.’ And yet here they are consumed by fire from God, for offering strange fire.

I find in Exodus 30:9 that there they were forbidden offering strange incense, but I do not find that they were forbidden offering strange fire. In Leviticus 6:13 and divers verses in that chapter we find that God had appointed that they should keep constantly the fire on the altar burning, and never to let it go out: Now that was, it seems, God’s intention that therefore they should make use of that fire, and that fire only. God would have them to pick out his meaning: God sent fire down from Heaven upon the altar, so in the latter end of the ninth chapter God sent down fire from Heaven, and gave them a charge to keep that fire on the altar constantly, and never to let it go out: so that it seems God would have them pick out his meaning, that because he had sent down fire from Heaven upon the altar, and gave them power to keep that constantly, God would have them therefore to understand, that what incense or sacrifice he would have the use of fire in, it should be only that fire and no other, though God did never say to them directly in these words, ‘You shall make use of this fire and no other,’ but God would have them to understand this. That’s their sin therefore in offering of strange fire.

. . .

The first note is this, that in God’s worship there must be nothing tendered up to God but what he hath commanded, whatsoever we meddle with in the worship of God, it must be what we have a warrant for out of the Word of God.

This speech of Moses’ is upon the occasion of the judgment of God upon Aaron’s sons for offering strange fire. They offered fire that God had not commanded. Hence I say that all things in God’s worship must have a warrant out of God’s Word. It must be commanded; it’s not enough that it is not forbidden. I beseech you to observe it. It is not enough that a thing is not forbidden, and you cannot see what harm there is in it. But it must be commanded. I confess that in matters that are civil and natural this may be enough. If it is according to the rules of prudence and not forbidden in the Word, we may make use of this in civil and natural things. But when we come to matters of religion and the worship of God, we must either have a command, or something out of God’s Word drawn from some command, wherein God manifests His will, either by a direct command, or by comparing one thing with another, or drawing consequences plainly from the words.

We must have a warrant of the worship of God. One would have thought that these priests offering incense to the true God, what hurt was there in taking other fire? But there was no command for it, and therefore it was not accepted. It’s true, there are some things in the worship of God that are natural and civil helps, and there we need not have any command: As for instance; when we come to worship God, the congregation meets, they must have a convenient place to keep the air and weather from them: now this is but a natural help, and so far as I use the place of worship as a natural help I need have no command. But if I will put any thing in a place beyond what it hath in its own nature, there I must look for a command. For if I account one place more holy than another; or to think that God should accept of worship in one place rather than in another: This is to raise it above what it is in its own nature.

So that when any creature is raised in a religious way, above what it hath in it by nature; if I have not Scripture to warrant me I am therein superstitious. It’s a very useful rule for to help you: If any creature that you make any use of in a way of religion beyond what it hath in its own nature, if you have not some warrant from the Word of God (whatsoever specious shew there may be in it) it is superstition.