The ego in each of us acts like a gatekeeper, or controller, who decides what will be accepted or rejected. Everything has to go through it and meet its approval. It acts like "the boss" who has authority and who gets to decide whether something is true or false.
We feed everyting through this "opinion maker", and if we decide not to agree with something we’ll push it away. This really is the ego’s arrogance in trying to be that which decides what the truth is. If it decides something is false, it is false, and will not be acceptable.
So with regards to ACIM, Jesus has written various things and all of them are true. It is our job to actually trust Jesus and trust what He has said is true. But our ego is not happy about that. It wants to decide whether it is true or not, or what it means. It will accept some things, and reject others. It will only "approve" certain things that meet its criteria.
But what we’re supposed to be doing is to trust that Jesus is being truthful, which means, instead of asking ourselves whether what he says is true or not, to put aside the function of deciding if Jesus can be believed, to "blindly accept it" through FAITH, and to THEN spend time figuring out how it MUST be true.
Do we trust Jesus? Is he always telling the truth? Does he lie? Does he contradict himself? Do we have faith in his word? Do we put aside "trying to be right" or "being the decider", and let Him be the authority? This is part of surrendering our ego role of defensiveness and reluctance and having a heartfelt trust and willingness to accept the truth. To accept it not because we even necessarily understand it, but because it’s coming from Jesus.
I trust that what Jesus is saying is true, and then try to learn how and why it is true. I accept it first, and explain it later. And where I fail to trust and want to decide what the truth is myself, those are my lessons in trust.
Jesus is the man.