Belief is an artificial tool that we use in place of knowledge. Belief attempts to establish something as true. Belief is a concentration or focus, a separated off state, in which that which has been "accepted as true" is allowed into the mind to be experienced as desirable, while that which is "outside" of that box is experienced as unwanted, wrong, false, to be rejected.
So then once we're in belief, we have a bias. We will only seem to accept a certain portion of the world, while rejecting the rest. And we'll then use this belief as a barometer for "reality testing" to determine whether things line up with it or not. If they seem to be in the category of acceptable or complementary, they are accepted as true. If they seem contradictory or outside the belief, they are considered false.
The problem with this isn't so much how beliefs work but that we actually use belief to believe things which, in the big picture, are simply not true. We side with lies, buy into illusions, take on beliefs and models and concepts which are incorrect, and then defend them to the death believing they are correct. But this then blinds us to the actual truth.
When we encounter the real truth, holding onto false beliefs, we're going to reject it and see it as wrong, false, evil and attacking. Unwanted and incorrect. Our allegiance to our belief will act like a defense mechanism, shutting out what we deem to be incorrect. But what we're shutting out can often be what is actually true, and now our belief is actually hurting us and keeping us away from the truth, all the while under the guise of having no reason to suspect it is flawed.
Particularly when we are confronted with something which is true, but which we don't believe in, we will simply deny it. It doesn't matter how clear it is, how true it is, how obvious it is, how much sense it makes, how reasonable it is, or anything else. Due to the fact we have this distortion in the mind steering us away from and towards some other truth, we'll keep defending our existing position against this new idea. And we'll see it as threatening and wrong.
The ego is doing this all the time. It's a belief system based on lies and denial of the real truth. So as we have these chunks of bullshit in our mind, which we believe are totally true and we are convinced of them, perhaps having spent a lifetime supporting and bolstering them, we will not be ABLE to WILLING to accept a larger truth WITHOUT letting go of the old belief.
Wrong beliefs HAVE to be questioned and recognized as being mistaken, before you will be willing to let them go in favor or something else. You won't be able to accept into your mind, what seems to be a truth which contradicts what you believe, unless you are ready and willing to let go of what you used to believe.
Your old belief will nag at you, tug at you, remind you of itself. It will pop up into conscious awareness asserting itself and saying "but what about xyz". It will try to continue to get your attention and keep your allegiance to it, because now as you threaten to abandon it it's going to come out of the shadows and be up for examination. It's like a little part of your mind, that's committed to this belief, worried that you are letting "it" go in favor or some other truth.
You can't accept the real truth WITHOUT letting go of false beliefs. The false beliefs will pull you away, make you doubt the truth, make you feel like you can't trust it, that there's something wrong with it even if you don't know what it is. False beliefs makes you suspicious of the real truth. Beliefs distort your perception and actually make things appear to be "wrong" even if they're not. They act as a filter and a magnet.
You could be presented with reality at all times. You could be surrounded with love at all times. You could be in heaven right now. You could be in the real world. You could be faced with the simple truth that nothing real can be threatened and the world is not real. But you will not see it, acknowledge it, accept it, or allow it to be true for you, thus welcomed, without your willingness to recognize it as correct. And the reason you don't believe it's true is because you believe SOMETHING ELSE which contradicts it.
So what you'll find happening is that very often as we come to the course, there are things the course says which are true, but which you do not believe. You will be presented with these things and will deny them. You will be like, no, that's not true. Even if it is explicitly spelled out, you will say "i don't believe that". You will hold up models and concepts that other teachers have come up with that are wrong, and believe them MORE than you will believe what the course is saying. You'll take your own conviction in your own rightfulness over Jesus's authority. And you'll reject the atonement on this basis.
Our false believing, our mistakenness, our confusion, our siding with things which are not true believing they are, all needs to be corrected. The mind's acceptance of what is true has to be brought in line with God's established truth, which you have NO SAY IN. You can only accept it as is, or reject it. But in order to reject it you're going to have to come up with a belief, which makes you feel that something ELSE is true. And that therefore, God's truth is not true.
All beliefs pointing away from what the course says verbatim must be mistaken. It does not matter who came up with them, who taught them, how convincing they are, or how many people believe it. If it says something that the course DOES NOT SAY, it is wrong and is taking you away from the truth. If you believe something that explicit statements oppose, it is wrong and needs correction. And so we have to learn to be willing to put Jesus's truth above that of ourselves, other teachers, and anyone's opinion, if we are to listen and truly learn what it is trying to tell us.
We have to throw out all false models and concepts and all archaic interpretaitons. We especially need to beware of taking an interpretation over facts. If we want God's truth, we need to learn what it is, recognizing we do not know.