A cult is an organization that takes away the divinity of Christ.
to name a couple
Mormonism
Jehovah Witness
That's not a definition of a cult used by people who generally study and research what cults are. This particular definition you've provided is a fairly idiosyncratic one used, primarily, in an informal way by modern Evangelicals.
A person who denies the deity of Christ is not a member of a cult, they are a heretic.
Cults can, and often do, include heretical or heterodox beliefs, but that isn't the defining feature of a cult.
The Church of Scientology is a cult, though because they do not claim to be Christians we do not categorize them as heretics. The Church of Scientology is a cult because as an institution it engages in cult behaviors, such as brain-washing, member control, thought policing, shunning, and in psychological and emotional abuse.
Heretical groups can be cults.
Groups that are nominally orthodox can also be cults.
And entirely non-Christian groups altogether can be cults.
So, for example, the LDS church probably shouldn't be identified as a cult, however the FLDS (Fundamentalist Church of Jesus Christ of Latter-Day saints) most certainly is a cult. Though both the LDS and FLDS are heretical.
Another example, Mars Hill based out of Seattle, WA when it was under the leadership of Mark Driscoll was a cult, though theologically they asserted the basic tenets of orthodox Christianity and was nominally Reformed in its theology. Mars Hill was a cult because of the way the leadership exercised abusive control over its members. Fortunately Driscoll's authoritarian rule over Mars Hill reached a breaking point and Driscoll was removed, and the central leadership of Mars Hill gave way to more independent congregations. I don't know the current state of affairs, but last I heard things were moving in, at least, a better direction.
Another example of a nominally orthodox cult would be the Exclusive Brethren.
If the only definition we give to a cult is a denial of the deity of Christ, then that would mean groups like the Church of Scientology which doesn't even claim itself to be Christian in any sense would not be a cult, neither would seriously abusive groups like the Exlusive Brethren be regarded as cults.
Cults are not merely heretical Christian groups, they are abusive and emotionally, psychologicall, or even physically and sexually abusive religious groups or institutions whose authoritarian control over membership pose serious risks and dangers to its members. C.f. Heaven's Gate or The People's Temple (aka Jamestown).
-CryptoLutheran