I remember, as a Catholic boy, thinking that Protestants were not true Christians. Then, when I became an evangelical Protestant, some fellow evangelicals told me that Catholics could not be Christians if they believed everything that Rome taught.
But things are more insidious now. Some conservative Protestants believe that Middle Eastern Christians who are being murdered for their faith might not be real Christians.
http://philosproject.org/persecuted-christians-middle-east-really-christians/