The term "radical" has a generally negative connotation these days, but a radical can be a positive thing also.  Jesus was very much a radical, at least from the point of view of the priests of the Temple and the Roman Governor, because his ideas limited their authority over people.  The same is true of Buddha. 
St. Francis is another well known radical Christian.  He rejected the materialist of his father and even many in the Catholic Church who opposed his philosophy of limited comforts, having only what was needed to service if you wanted to pursue a spiritual life.  Martin Luther King Jr. and Gandhi were also radicals of a sort, but most would agree that it was a positive type of radicalism.  In fact, when people become complacent and comfortable with their creature comforts and material goods, it usually takes a radical to shake things up and get people to turn toward a more spiritual path.