PITH

the essential or central part

the choicest or most essential or most vital part of some idea or experience
SYN - center, core, essence, gist, heart, marrow, meat, nitty-gritty, substance

"He knew that this silly banter was useless and that they needed to get to the pith of the argument."

"It was the pith of their trip and the fact that someone was trying to taint this event led them all to take up arms against the perpetrator."

"The problem was the pith of their ideology was just plainly false - it couldn't even be true."