Arianism is an ancient theological error that appeared around the year 320.  It taught that God could not appear on the earth, that Jesus was not eternal and could not be God.  Additionally, it taught that there was only one person in the Godhead: the Father.  Jesus, then, was a creation.  It was condemned by the Council of Nicea in 325.

The Jehovah’s Witness cult is an equivalent of this ancient error, though not exactly.  Please see Heresies for more information.