I don't fully buy it. If I'm God, shouldn't I know I'm Him? If I ever did know I was and could be absolutely certain about it, things would be a lot different around here, that's for sure.
Totally understandable. It's actually quite dangerous to bring up this sort of thing since it can be misused to invalidate people, hence why I'm a bit nervous doing so.
But just for fun, I'd like to have a go at answering your question since it's a good one. Firstly, this is what the aforementioned
book has to say:
"Of course, there is no way for you to not be who and what you are—you simply are that (pure, creative spirit), have been always, and always will be. So, you did the next best thing. You caused yourself to forget Who You Really Are.
Upon entering the physical universe, you relinquished your remembrance of yourself. This allows you to choose to be Who You Are, rather than simply wake up in the castle, so to speak."
The claim that we choose to come here (as outrageous as it can seem when in the throes of suffering) is also an insight regularly delivered by near-death
studies. For example, that of
Thomas Sawyer:
"The goal is true self-realization. Self-realization is to know that you are a soul, a part of God, yet also the Whole."
The other part of your question asks whether it is possible to be absolutely certain about this stuff. I understand that the answer is yes, but it is something that gets very tricky to talk about. There are many experiences which can provide a profound glimpse into our true nature (things like mystical experiences, some psychedelic experiences, some NDEs, etc.). There's also the angle of rational argumentation from philosophies like Advaita Vedanta aiming to correct the body-identification that is at the root of our suffering.
But the tragedy of circumstance is that people who have achieved the big shift in identity have no way of directly conveying how radically different their perspective is (and they often view the world as perfect as is, thus have no motivation to convince others of anything). They may even appear as
normal functional people to the untrained eye.
So we have a situation where the unenlightened person sees the enlightened person either as just another human, or as someone holy to glorify, whereas the enlightened person simply sees other people as
themselves. And only the latter sees things with clarity and certainty.
People who have at any point in their lives had a direct insight will not need much convincing that this is the true nature of reality. For them, it becomes a question of how to discard the false self and all of its trouble, with an inevitable deep dive into a world of spiritual texts, practices and teachers. Without any inkling that this
might be true, however, it's hard to see anyone being driven to make those efforts in this lifetime.