The title is a bit over dramatic but, per the title, if you could contribute with one piece of knowledge to a book that every single individual should learn from in order to kickstart a civilization, what would be yours?
My personal choice would be the process of soap making, from scratch.
There will always be religion, especially if and when the civilized world ends.
A better way would be just to remind everyone that there were countless religions before and that they were all man made, corrupted and fell apart after a certain amount of time.
Remind everyone that there is no one true religion because there never was one before, there isn’t one now and there never will be be one.
But I’m afraid that as much as we’ll try … people will always be dumb enough to want to believe in fairy tales, an after life, eternal bliss / hell and that one group is better than another.
“Yes, all the religions before were false, but ours is the right one!”
I think in a way religion was a necessary component in managing populations.
Without education humans will naturally look to “organised luck” or supernatural deities for meaning. That can be used as a kind of carrier wave to disseminate basic information.
“God says you should clean your cock and balls once a day in order to bring good luck”