If you could teach everyone in the world one concept, what concept would have the biggest positive impact on humanity?
I would at least try to teach them that their belief or opinion (be it theological or otherwise) is not necessarily better than the equally dearly held belief of others. That unshakable belief in superiority shuts down any sort of chance conversation or compromise.
Every human should do LSD once.
Just kidding, kinda....I would say love and forgiveness.
I would like to say how to manage their retirement fund but given that 9 out of 10 humans on earth live in the developing world it doesnt matter to the bulk of humanity. Kinda hard to start talking about portfolio re-balancing with a 3rd world rice farmer who sleeps with their ak-47.
Maybe basic sewing?