When the colonies formed the U.S.   they all had the common footing of faith in Christ Jesus.   It is through Jesus that they knew what freedom was This fundamental consensus provided the glue by which they could come together in spite other differences.   

When the South seceded it was because the North breached the union on the basis of freedom.   The freedom that was breached was at the state level.   The federal government determined that there would be no more slave states added to the union.   The South knew that this decision would eventually erode all states rights (they were correct).   

So one of the 2 pillars of the union was broken and secession resulted.   

Today, the leadership of California (and we must be careful to distinguish their leaders from the people of California) does not tout California as a Christian state.   They do not tout California as a freedom-loving state.   (They actually want to secede because they are fascist).   They tout themselves as elitist socialists.   They have no immigration control.   They will likely go bankrupt because of socialist policy and government programs.   

They are ignorant of the implications of our president's America First policy.   To secede would mean there is no free trade between them and us.   It would mean dependence on their own tax system to support their government programs.   It would mean that Californians would not be able to travel into the U.S.   without passport or visa.   It would likely mean a wall around California to keep the people they have allowed inside California from streaming across the U.S.   border.   

Bottom line: The flakes that would suggest such secession would of course run California unto 3rd world status in short order.