God gave man a brain to think, discern and judge. Jesus warned of false prophets and cults. Paul wrote about it extensively in the first century. It is not hard to look at the core central beliefs of Christianity and discern cults and false prophets. We are called to do that.
This "defining" you seem to have a problem with is defined by Christ and is based on a common standard called the Bible. I am not defining anything when it comes to the core gospel which is to accept Christ as your savior through faith, repent and you will be saved. This is so broad that it is accepted by all mainstream Christians and Mormons. There will always be theological disagreements and sinful man will never agree on anything. That's why it's critical to focus on the core as taught by Christ and not throw the baby out with the bath water.
That was what I believed, yet you said I couldn't have been a real Christian.