The early Christians didn't break away from Judaism. They were kicked out. The Apostle Paul always went to synagogue as an early believer to tell Jews about his faith. Before Paul, the early believers in Jerusalem met in the Jewish temple in Jerusalem teaching about Jesus - the son of God not just a man. Jesus had demonstrated who he was in act and word and he gained followers because of it. Upon his crucifixion and subsequent resurrection, his followers (Jews) proclaimed him alive and resurrected precisely because of what they had witnessed live and because they were Jews which commanded them not to lie. These Jews and early Christians took the commandments of God very seriously and if Jesus were a fraud in their eyes and had not resurrected, there is no way they would have proclaimed otherwise. These early believers - the ones who walked with Jesus - were persecuted for their beliefs and died for them. They would never have put their lives on the line for a lie.
The early believers were certainly viewed in negative terms by Jews and then the Pagans of the Roman Empire. Being negatively viewed by others is not mark of being a cult although the word "cult" is really just a word from Latin meaning "worship" just as Fetus is from the Latin for Baby.
So to be technical, Romans probably referred to early Christians as belonging to the "Cult of Jesus" but early believers just called themselves People of the Way or Anointed Ones (the Word Christian is a transliteration from Greek and just means "anointed" ). The word Christian isn't even a translation.
What we mean by "cult" today is altogether something else. If you want a definition of a "cult" in today's language it is characterized chiefly by CONTROL. This characteristic is true of virtually all modern cults - control of others including their thoughts. A true Christian struggles with doubts from time to time - if they are honest - and that's because of the world we live in and because their relationship is weak. The early Christians were so strong in their faith that they put their lives on the line and did it without fear. Those of us who call ourselves Christians today have a ways to go in order to equal the early believers.
So it is possible for a false religion to be a cult or not a cult. It is also possible for a Christian group to be a cult or not a cult. False belief is not enough a reason to call a group a cult. Your first clue for a cult is control - i.e. somebody forcing others through physical force, emotional threats, fear etc. A cult can be as small as a family or as big as a nation (Germany - Nazis).