What are the main beliefs held by the Jehovah’s Witnesses that are contrary to the beliefs of Christians from at least Nicene Creed 300 AD to 1800 AD?

I know very little about Jehovah’s Witnesses other than that in general they reject traditional Christianity and think that only through their new teaching, can people understand the truth, whatever that is. What are the central departures that they take from the gospel message believed by Christians from Nicea up until their founding in the late 1800s?