I went to a Christian high school and I was informed about different religious groups such as the Mormons and the Jehovah's Witnesses. I was taught what was wrong with them and how to witness with them. The faculty wanted to warn us about them because those groups preach a false gospel.
If all you know is what is wrong with some group, you can start to see that everywhere - including in places it doesn't fit. All the wrong doctrines have contexts amongst the right ones. Some are more blatant, others far subtler than many can grasp.
Take medicine, as an analogy. If all you are taught is symptoms of disease, you'll see disease everywhere - even in places it doesn't exist. A baby, for example, can run a fever when cutting teeth. Does that mean they are sick? No, it simply means the body is reacting to something (an event, in this case). But if someone is taught to treat every fever the same way, they won't understand it properly. All they see is the bad (fever) without the broader (good) context (cutting teeth). To be picky, the fever itself isn't really bad - it's actually the body's way of fighting invaders.
You need to know how
everything fits together before you can properly deal with defective operation.