Most students of the Reformation recognize that Martin Luther discovered (more accurately re-discovered) the doctrine of justification by faith alone and that Ulrich Zwingli discovered the symbolic interpretation of the Lord’s Supper. At least, these Reformers popularized those doctrines.
But who discovered the regulative principle of worship? No, it wasn’t John Calvin or John Knox. It was actually an Anabaptist. Surprise!
The earliest statement of the regulative principle of worship that I have found in the Reformation era is in a letter written by Conrad Grebel (the ringleader of the Zurich Anabaptists) to Thomas Müntzer on September 5, 1524.
Speaking on behalf of the Zurich Anabaptists, Grebel said to Müntzer, “That which is not taught by clear instruction” we regard as forbidden, just as if it stood written, “Thou shalt not do this.”
This principle is applied in the letter to various matters of worship including infant baptism. “Nowhere do we read that the apostles baptized children with water. Consequently, in the absence of a specific Word and example, they should not be baptized.”
Likewise, in a dispute over infant baptism with Zwingli, the Anabaptists argued, “Children are nowhere in Scripture commanded to be baptized, nor is it anywhere said that Christ or the apostles baptized children;” hence, it is a man-made tradition that “ought to be done away with as an abuse, as other papistical abuses have been done away with.”
Grebel apparently discovered the regulative principle in the writings of Tertullian.
When the works of Tertullian were published in 1521, Grebel was one of the first to study them. In De Corona, which Tertullian wrote around the year 211, we find the story of a certain Christian soldier, who refused to wear the laurel crown on the accession of the emperor Severus. This led to the soldier’s imprisonment.
Some Christians argued that the soldier was making a big deal out of nothing, a mere matter of dress. “After all,” they reasoned, “we are not forbidden in Scripture from wearing a crown.” Tertullian, on the other hand, wrote De Corona in defense of the soldier’s actions.
To be sure, it is very easy to ask: “Where in Scripture are we forbidden to wear a crown?” But, can you show me a text that says we should be crowned? If people try to say that we may be crowned because the Scriptures do not forbid it, then they leave themselves open to the retort that we may not be crowned because Scripture does not prescribe it. But “Whatever is not forbidden is, without question, allowed.” Rather do I say: “Whatever is not specifically permitted is forbidden.”
These two opposing principles—whatever is not forbidden is allowed (on the one hand) and whatever is not commanded is forbidden (on the other)—reappear in the sixteenth century debates on worship.
Both the Calvinists and the Anabaptists employed the latter principle, but the two groups had different criteria for what constituted biblical warrant to justify liturgical practice.
Specifically, the Anabaptists had a narrower understanding of biblical warrant and, therefore, a more restrictive version of the regulative principle than the Calvinists had.
“Direct biblical warrant, in the form of precept or precedent, is required to sanction every item included in the public worship of God,” claimed the Anabaptists. Therefore, they rejected infant baptism, for instance, because of the absence in scripture of any clear command or example to justify it.
On the other hand, Calvinists recognized that biblical warrant could be established, not only by precept or precedent, but also by biblical inferences or, as the Westminster Confession says, deductions by good and necessary consequence.
As James Bannerman explains,
The doctrine of the Westminster Standards [WCF 1:6] and of our church is, that whatsoever is not expressly appointed in the Word, or appointed by necessary inference from the Word, it is not lawful for the Church to exercise of its own authority to enjoin; the restriction upon that authority being, that it shall announce and enforce nothing in the public worship of God, except what God himself has in explicit terms or by implication instituted.
 Dr. Hughes Oliphant Old tipped me off to the Grebel-Tertullian connection.
 Robert Dick Sider, ed., Christian and Pagan in the Roman Empire: The Witness of Tertullian (Washington, DC: Catholic University of America Press, 2001) 120.
 J. I. Packer makes this comment about the Puritans, but in our opinion, it is more descriptive of the Radical Reformers; see Packer, Among God’s Giants: The Puritan Vision of the Christian Life (Eastborne: Kingsway, 1991) 326.
 James Bannerman, The Church of Christ (Edinburgh: The Banner of Truth Trust, 1974) 1:340.