How do I make a before_action to run on all controllers and actions except one?

You can use params[:controller] to detect controller name of request.

class ApplicationController < ActionController::Base
  before_action :authenticate_user!
  AUTHENTICATE_USER_EXCEPT_CONTROLLERS = ['controller_names_you_want_to_exclude']
  ...

  def authenticate_user!
    unless AUTHENTICATE_USER_EXCEPT_CONTROLLERS.include?(params[:controller])
     super
    end
  end

end

What you have to do is to set autheticate_user! on all controllers like that :

class ApplicationController < ActionController::Base
  before_action :authenticate_user!
  ...
end

And then on your HomeController you do that :

class HomeController < ApplicationController
  skip_before_action :authenticate_user!, only: [:index]
  ...
end

Hope this will help you !