Hint: the body of Christ we call church.
Kevin DeYoung defends a wide berth for Reformed Protestantism and quotes Herman Bavinck for support:
In particular, Bavinck claims, “From the outset Reformed theology in North America displayed a variety of diverse forms.” He then goes on to mention the arrivals of the Episcopal Church (1607), the Dutch Reformed (1609), the Congregationalists (1620), the Quakers (1680), the Baptists (1639), the Methodists (1735 with Wesley and 1738 with Whitefield), and finally the German churches. “Almost all of these churches and currents in these churches,” Bavinck observes, “were of Calvinistic origin. Of all religious movements in America, Calvinism has been the most vigorous. It is not limited to one church or other, but—in a variety of modifications—constitutes the animating element in Congregational, Baptist, Presbyterian, Dutch Reformed, and German Reformed churches, and so forth” (1.201). In other words, not only is Bavinck comfortable using Calvinism has a synonym for Reformed theology (in this instance at least), he also has no problem affirming that Calvinism was not limited to one tradition alone but constituted the “animating element” in a variety of churches. Calvinism, as opposed to Lutheranism, flourished in colonial America as the typical orthodox, Reformational, sola scriptura-sola fide alternative to the various forms of comprised Arminianism and heterodox Socinianism.
The problem with this historically speaking, for starters, is that Lutheranism did precede Calvinism and so you could conceivably attribute all the variety of Calvinism to Lutheranism as the original Protestantism. Granted, the lines of continuity between Reformed Protestantism and the North American colonial churches were stronger than with Lutheranism. But that is much more a function of British Protestantism and what happened to Calvinism (or what didn’t) within the Church of England, the Union of England and Scotland, and the Puritans. British Protestantism turned Calvinism into a proverbial hot house of Calvinisms. This was not the case among the Dutch Calvinists who planted Reformed churches in North America. The colony of New Netherlands actually excluded Quakers and Lutherans, and enjoyed much greater uniformity than the Old World Dutch were capable of enforcing. Remember, the Netherlands, despite Dort, welcomed Descartes, Spinoza, and Anabaptists.
But aside from the history, the question is one of arbitrariness. If John MacArthur can exclude charismatics from being Reformed even though he doesn’t belong to a Reformed church, or if The Gospel Coalition can set up a tent broad enough to include disciplined Southern Baptists and wobbly PCA ministers, Calvinism, like evangelicalism, becomes simply what pleases the excluder/includer. Add to that the reality that conservative Presbyterian and Reformed communions invested great energy and resources to distinguishing themselves from communions, like DeYoung’s, those that are Reformed primarily in name rather than substance, and you begin to see why some Reformed Protestants are eager to give coherence to their wing of Western Christianity. I don’t mean that as a cheap shot. But so far, folks like MacArthur and the Gospel Allies have yet to acknowledge the hard work done by Reformed Christians to defend and maintain the ministry of word and sacrament within disciplined (read Reformed) churches. We had thought the task of reforming the church was arduous and long, but now you hold a conference or set up a blog and — voila — it’s Calvinism.
Dictionaries revise definitions all the time. But users of words and grammarians don’t approve of the revisions. The question comes down to whose pay grade it is to establish Calvinism’s meaning. Celebrity pastors? Parachurch agencies? Or church councils? I’m pretty sure I know how Calvin, Bucer, Knox, and Ursinus would vote. Do they carry as much clout as John Piper? As Bud Dickman is wont to say, “well. . .”