ACIM leads off with this:
"This is a course in miracles. It is a required course. Only the time you take it is voluntary. Free will does not mean that you can establish the curriculum. It means only that you can elect what you want to take at a given time. The course does not aim at teaching the meaning of love, for that is beyond what can be taught. It does aim, however, at removing the blocks to the awareness of love's presence, which is your natural inheritance. The opposite of love is fear, but what is all-encompassing can have no opposite."
It is saying that we are REQUIRED to take this 'course', and that there is some kind of 'curriculum' that is not optional.
Many believe this means that we each have an individual curriculum, which we do (a script).
But what it is also points to, is that there is a set of metaphysics which describes a kind of pre-set or structured layout to existence, describing the mechanics of the dimensions of the dream, how things are 'set up', where the truth IS (God), what the truth is not (ego), and what steps are necessary in order to find true peace.
This layout, or 'plan', if you will, is not optional. It is not something you can interfere with. It is not something you get to determine. It is a set of 'rules' or 'law' by which everything is governed. There is a law of cause and effect. There is a law that what you do to others you do to yourself. etc... these are non negotiable.
The ego thinks it has the power and freedom to establish the rules, to determine where the truth is, what laws govern existence, that you have an 'option' of living without God, that you can just make everything up to be the way you want it, that you can find truth in what is false, that you can live without God (Life), etc... the ego has this fictional idea that it is 'above the law'.
The christians would say that the ego (as the devil), is the law-breaker which thinks it can 'sin' by breaking God's laws, i.e. defying the curriculum. But we're all doing this whether we're christians or not.
The curriculum is the metaphysics. The curriculum is God's plan. The curriculum is the 'how' and the 'requirements' and 'conditions' necessary in order to return to awareness of God. It's like, a set of laws that you have to accept and agree to, if you want to play-nice with the establishment.
As such the metaphysics determines the 'way to God', so all that's left really is the timing of your willingness to ACCEPT the path to atonement, by surrendering the ego idea of self-control and the illusion of choice.