The two sound the same. “Radical” means going to the root, “Fundamental” means going the the foundations; the meaning is almost the same. However, there is a big difference here. “Fundamentalist” most often refers to Churches that are more or less mainstream in the USA today. “Radical” refers to the little-known Radical Reformation. Everyone has heard of “The Reformation” and its key founders, Luther and Calvin, but these men were not truly radical in their views. Much of what they said and did simply followed the Roman Church they departed from. The Reformation they presided over is also called the Magisterial Reformation to distinguish it from the Radical Reformation of men like Wycliffe, Hus and Simons. The word “Magisterial” is used to refer to the interest in state power that Luther and Calvin shared, something definitely not espoused by the Radicals; another of the key differences is in their respective approaches to the use of violence. Radical Christianity seeks to conform more closely to the New Testament. Fundamentalist Christianity is essentially in the Magisterial vein, and lacks the interest in the Fundamentals of Christianity that its name suggests. See also Calvin the “Christian” Murdering Blasphemer.