What's the Difference Between a Cult and a "Legitimate Religion"?

In this interesting letter to the editor Jeffrey Kline is found bewailing cultist mind control. Does he not see the equivalent mind control of an enculturated "legitimate religion." Didn't every single religion start out as a cult in the first place? Then what causes one religion to eventually become a "legitimate religion"? Time? Numbers? What is a "legitimate religion" anyway?