If your definition of "cult" = "somebody who does not believe Sola Scriptura", then yes (and vast majority of professing Christians) are "cultists". I passionately do not believe in Sola Sciptura.
I don't believe that any major Protestant denominations, or the Catholic or Orthodox churches state that they elevate any book above the Holy Bible in authority. (Not to say that I can speak for all fringe groups out there.)
Protestants typically object to the Catholic doctrine of ex cathedra (the Pope speaks infallibly "from the chair"). In fact, this was a major reason for the Protestant Reformation. I strongly object to this doctrine, but it does not constitute elevating any written books or scriptures above the Bible.