Arianism (Ἀρειανισμός Areianismós) is a Christological doctrine first attributed to Arius (c. AD 256–336), a Christian presbyter from Alexandria, Egypt.
There is no other doctrine that has been so hotly debated in the last 2,000 years. Christians have literally killed Christians for believing differently. All other Christian doctrines have several different views. This is not the case with the Trinity doctrine, if one ever so slightly gets of course from the orthodox view, it is tantamount to heresy.