"People don't have ideas, Ideas have people" - While perhaps not a direct quote of Jung, this phrase is often attributed to him. For me, it speaks to how people can allow certain ideas to 'possess' them, often narrowing and restricting their vision of the world or themselves to fit within the limits that the idea and all it's supporting belief's dictate.
Being involved in the realm of spiritual development, belief is something that is very important to me, realizing that it is our beliefs that shape us, bind us or free us. The way in which we experience and interact with the world and how we view ourselves is largely if not entirely a product of the underlying beliefs we maintain. Most of these beliefs are built up over time through repeated or intense experiences, like rivers carving grooves over land. They are interdependent and interlocking in nature, such that if a core belief begins to unravel it can take the whole structure down with it. What at a superficial level may appear to be a single belief to our conscious awareness is actually only the visible part of a much deeper structure. Most of these underlying beliefs go entirely unnoticed until the ground starts to shake, much like our unawareness of a particular organ until it starts to malfunction.
Often this can be a good, and even necessary thing, though it may present itself as painful and destabilizing. Obviously there's nothing wrong with being passionate about ones beliefs. The danger comes when one walls oneself inside their belief system, blocking out any input from outside, or instantly negating anything that doesn't correspond with their preexisting system. This creates a very narrow view on reality, often presenting only one possible interpretation to what is actually much more complex. The person ceases to realize that what they now consider as fact is actually their interpretation of a given situation and is a product of their beliefs, assumptions and the way in which they have chosen to perceive the world.