Machen’s reasoning here was an extension of the Regulative Principle. In the Presbyterian and Reformed tradition this principle has typically been applied to public worship. It teaches that we may only worship God as he has commanded us to worship him in his Word. People who hear this doctrine for the first time often understand it as overly negative and restrictive, as if we have no freedom in worship. Though the Regulative Principle does limit what we may do in worship, just as important is what it teaches about liberty of conscience and the Lordship of Christ. As the Westminster Confession of Faith teaches (20.2), “God alone is Lord of the conscience.” To bind the consciences of believers on the teaching of Scripture is to recognize and extend Christ’s Lordship. But to do so only on the basis of human wisdom or preference is to usurp his rule.
This principle is what separates Presbyterians from other Protestants. Unlike Lutherans and Anglicans who believe that churches may do whatever God’s word allows, Presbyterians and Reformed teach that churches may only do what Scripture commands; hence the name Reformed, “reformed according to the Word.”
The Regulative Principle applies not only to worship, but to all aspects of the church’s life and witness. Unless the church can find a clear warrant from Scripture for a particular teaching or practice it may not speak or act. Otherwise it runs the risk of binding the consciences of believers and usurping the Lordship of Christ. In this broader sense the Regulative Principle is only a variation on the formal principle of the Reformation, namely, “sola scriptura.” The Reformers believed that Rome had substituted the word of man (i.e. the papacy) for the Word of God. John Calvin grappled with just this issue when he responded to the argument that he should submit to the laws of the Roman church even if they were unjust because God commands that Christians submit to the powers that he has ordained. Calvin responded that it was not a question simply of enduring “some grievous oppression in our bodies.” The real issue was “whether our consciences shall be deprived of their liberty, that is, of the benefit of the blood of Christ.” According to Calvin this was no trifling matter. “No necessity ought to be imposed upon consciences in things in which they have been set at liberty by Christ,” he wrote, because without this liberty man could have no peace with God. “If [believers] wish to retain the grace which they have once obtained in Christ; they must submit to no slavery; they must be fettered by no bonds.”