Spiritual power is a very real thing and very dangerous when misused. That is one of the reasons why spiritual truth must be taught gradually and preferably under the supervision and guidance of a good spiritual teacher.     
    Let's look at a material example to understand what I am talking about. 
    You have an adult student who is studying to be a demolitions expert. He has been shown to have a stable mind and is reasonably intelligent. You feel it is reasonably safe to teach this person how to make explosive, how to determine how big a charge to use, how to figure out where to place the charges to best bring down the building or bridge, etc. Of course, there is always some risk that the person is unstable, or secretly a sympathizer with terrorists, but you have done as much as you reasonably can to teach this subject with safety. 
    But what about a ten-year-old kid who likes to blow things up for fun. Would it be reasonable for you to teach him the same subjects? Of course not. You might teach him some things about safety regarding explosives and why kids should stay away from them and never treat them as toys (although many still do on July 4^th), but you would not teach him how to decide how much dynamite to use to take down a building such as that school he goes to. So while you can teach the kid something, you can't teach him the same things you taught to the responsible and stable adult.
    And what about an adult who has a criminal record and has been convicted of starting fires, cruelty to animals, and bullying others when he was in school. He has also spent much of his life in and out of mental institutions and, while he is free now, is still considered to be mentally unstable and on drugs to control his fits of rage. Is this a good student to teach demolition to? To trust to have explosives with him, or with the knowledge to make them and use them? Obviously not. It would probably be safer to teach that ten-year-old than this guy.
    Now these three example are fairly obvious (I hope!) but, in real life, it is rarely that cut and dry. Nevertheless, the point is made. But what does it have to do with spiritual training? A lot. As Manly and many others have pointed out, spiritual knowledge is a powerful thing and, when put in the wrong hands and misused, it can cause considerable harm. The despotic Roman emperor Nero and Adolf Hitler are just two examples of people who managed to learn some spiritual knowledge they shouldn't have and were able to use it for evil purposes. Of course, those who misuse spiritual power will see that power taken from them, but they can do considerable damage before that happens. For that reason, advanced spiritual teachings are only given to those a spiritual teacher deems ready and capable and are never published for the general public.