"The Wall is not just ice and stone," Benjen Stark tells Bran and Meera in GoT's season 6 finale. "Ancient spells were carved into its foundation, strong magic to protect men from what lies beyond. And while it stands, the dead cannot pass."

So why the hell was Jon even worried? Why did he rally people to help fight them? HUGE plot hole. They had to get the zombie dragon to demolish part of the wall later to fix their mistake.