I have always heard it like this.
Cults are offshoots of a religion. While SECTS and DENOMINATIONS hold to core tenets of a religion, but may be very alien in terms of doctrine, CULTS are almost the opposite. Despite doctrinal similarities, they reject the core tenets of a religion.
Christian cults often have the rejection of the Trinity as their marker.
The problem with your definition is that a cult is an offshoot of a religion, is that all religions therefore started as cults. Christianity could be thought of as a cult of Judiasm. Protestantism could be thought of as cult of Catholicism. Latter-Day Saints (Mormons, of which I am one) did not break off from another church, so it could not be considered a cult according to your definition.
Also, as a believer that Christ is the literal spiritual and physical Son of the Eternal Father (Almighty God) as opposed to Trinitarian view where Almighty God, Christ and the Holy are locked in a three-way personality disorder, I do not believe that Trinitarian acceptance is required for Christian belief.
Trinitarianism was coerced in 325 ((Nicea), and settled by state execution by Emperor Tehodosius (edict of thesolonica).
Since Christianity pre-dates trinitarisanism , by your definition, trinitarianism would be a cult of Christianity.