The Regulative Principle of Worship (RPW) simply defined is this: Whatever God has not specifically commanded in worship is forbidden.
The Old Testament is full of either implicit or explicit condemnation of Judah and the Israelites on the basis of false worship. It is the fundamental reason for their downfall. In fact, if you read the 1st Chapter of Romans you can see that man either worships God as He is and is thankful to Him as Creator or he turns to idolatry. Idolatry leads to a “giving over” to folly, which, in turn, leads to depraved actions – a downward spiral of unrighteousness. But it all begins with false worship.
To ask “Where has God told us not to worship Him except as He has commanded?” Try Exodus and Leviticus for starters. Implicit in the details is a reflection of the fact that God expects to be approached a certain way and that sacrifices before Him will be conducted a certain way.
Why not simply broad brush the whole thing and specify that animals are to be killed, Priests appointed, etc? For one thing, it ties back to the nature of man as outlined in Romans 1. The idolatry of man is such that, even with such detailed commands, man invents ways to even goon up specific commands.
Read 1st and 2nd Kings. What is the sin of Jeroboam? For political pragmatism, he sets the nation of Israel on a course of idolatry that they never turn from.
Read Jeremiah. It’s filled with references that state: “…which I had not commanded nor did it come into my mind….” Idolatry is not simply something God has forbidden but is referred to as something which He has not commanded.
Read the whole book of Amos. Understanding how they are worshipping (after reading 1st and 2nd Kings) sheds light on why they’re being condemned. It’s like Romans 1 being lived out in the Northern Kingdom. Idolatry and social injustice are simply two sides of the same coin. People were actually stricter in their religious observances than the Law required. The only problem is that they weren’t at the Temple.
In fact, as I was teaching Amos recently I realized that two men looking at the Southern and Northern Kingdoms and watching two worshippers from North and South would have been hard pressed to tell the difference. If I’m Joe the Ephraimite and grew up worshipping at Bethel, my worship externally looked precisely the same as Harold the Benjamite who’s bringing his sacrifice to the Temple in Jerusalem. Perhaps the same Words were being spoken as they place my hands on the sacrifice. Perhaps they were both scrupulous about the Sabbath. The only thing that separated them was geography. Post-modern man would scoff at any notion that they’re any different on such a basis.
But God commanded worship at His sanctuary and not at the high places.
It’s pretty hard, in the end, to separate God’s prescriptions for worship from His prohibitions against the way man commits idolatry.
Because of the sinfulness of the human heart. If we’re not getting our ideas on how to worship God from Him in His Word then Romans 1 declares that our natural inclination is to invent idolatrous ways to do so.