Meaning, forgiveness is a fixation on the truth that Christ is real and nothing of the ego is real. It fore-gives by having a purely forgiving attitude so that there are no errors introduced into your own perception, so that you do not see sin as real. This means you are overlooking the ego and not believing in it. You are not supposed to believe the ego even really exists!
If you fail to do this, if you are not truly purely forgiving, which occurs in the Atonement and the acceptance that nothing "real" has happened, therefore innocence prevails, then you will enter into a psychological condition which Jesus has named "judgement".
Judgement becomes your attempt to evaluate or justify WHY you did not forgive, by explaining using various other reasons what the cause is of what you're now seeing. Since you're making a perceptual error and are hallucinating errors, you will see sin as real and will attempt to explain why it is real by judging, ie evaluating, fake reasons.
Also judgement means that you are attacking, are condemning, are attempting to give reality to illusions, and are seeing someone as less than holy. This then takes on and accounts for all forms of what you think of as judgement, which is a judgemental attitude, but really encompasses the ENTIRE ego thought system. Every part of egoic perception is judgemental because it's a split-mind thought system of self-attack and condemnation. It points a finger of blame to scapegoat and enter denial, and finds someone guilty.
The belief that sin is real is judgementalism and is the opposite of forgiveness.
A major mistake all egoic minds make is believing that one part of the ego mind is holier than another, and then pits that one part against the other. One part is called holy and the other part is called evil. And then there is judgement against those that are considered sinful, even though the entirety of that attitude is judgemental. You see people in Christianity doing this where they become very polarized and anti-sin, waging a "spiritual war" against the devil, because they believe it is REAL. But it's not just Christians that do this, everyone with an ego does this.
You WILL be judgemental if you are not forgiving, and the only sign of true sanity is One who has absolutely no belief in "real sin" whatsoever. That mind, Holy Spirit's thought system of real truth, demonstrates and proves that there is no real ego at all. Nothing has happened. The atonement.
People will then say, if you want to do the Course correctly, you need to not be judgemental. So then people repress their ego and hide their judgements. If they believe sin is real they will continue to judge on some level even if they don't express it. And it will be an unconscious self attack. You can't just "stop judging" in expectation that this will solve everything, because the only reason you're judging is because you believe the world/body/ego/sin is reality or truth. You could take that path IF it would lead you to the insight that NONE of that is real, but I don't generally think that's happening for most people.
Instead, focus on forgiveness which is the true undoing of judgement, which must lead all the way to the Atonement if it is to completely uproot the core belief about the separation really happening. Only then will judgement cease, because judgement is really an EFFECT of non-forgiveness, not a cause in its own right.
P.S. I don't mean this post to be a "judgement" about judgemental Christians. Not all Christians are judgemental by any means. But they do often present a good example of this happening due to their belief in real sin and can't help but be "against" sinners. Some of the most intensely judgemental people I've met claim to believe in Jesus with the most gusto and passion, yet were horrible to be around and so incredibly righteous. Their personas were huge and they were in such denial. This doesn't mean Christians can't be truly forgiving and not believe in sin, just that many don't get that far. This is the lower level of expressing Christ's truth and is an attempt to express it within the context of the ego being real, so is really a major "level confusion". Christians can become Christ if they can let go of a belief in real sin and the entire package of evil/hell/sickness/death/vengeful God that accompanies it.