So, I've been told by Earnshaws Theorem that it's impossible to trap a charged particle using an purely electrostatic configuration. However, this doesn't seem to make intuitive sense. If I were to create a sphere whose surface consisted purely of positive charges, evenly distributed over the surface and then placed a single positive charge in the middle, would that not be trapping a particle in an electrostatic configuration? There would even be a restoration force as if the particle was "nudged" slightly, the net force would be to push the particle back into the center. I'm mostly looking for an intuitive answer. Have I missed something? Is this not defined as electrostatic?
A mathematical explanation would also be appreciated alongside an intuitive one.