He portrays [mercy] as an unsentimental though humane experience … because it means facing up to the truth about what you have done and who you are. The theistic insight is that this truth can only be seen when you are confronted by the divine. To meet God – or Aslan, as Lewis has it in the Narnia stories – is "to meet someone who, because he has freely created you and wants for you nothing but your good, your flourishing, is free to see you as you are and to reflect that seeing back to you".
In other words, to see yourself as others see you might be discomforting but it will also always be skewed by the distorting lens of their self-interest. To be unmasked as God sees you is painful because purgative, but is also a path to true liberation. It is merciful because without it we are left in a citadel of self-deception, life's energies being sapped and wasted on bolstering self-regard.