God is Never Obligated...

”Suppose ten people sin and sin equally.  Suppose God punishes five of them and is merciful to the other five.  Is this injustice?  No!  In this situation five people get justice and five people get mercy.  No one gets injustice.  What we tend to assume is this: if God is merciful to five he must be equally merciful to the other five.  Why?  He is never obligated to be merciful.  If he is merciful to nine of the ten, the tenth cannot complain that he is a victim of injustice.  God never owes mercy.  God is not obliged to treat all men equally.  Maybe I’d better say that again.  God is never obliged to treat all men equally.  If he were ever unjust to us, we would have reason to complain.  But simply because he grants mercy to my neighbor gives me no claim on his mercy.  Again we must remember that mercy is always voluntary.  “I will have mercy upon whom I will have mercy” (Ex 33:19; Romans 9:13). There are only two things I ever receive from God – justice or mercy.  I never receive injustice from His hand."

-R. C. Sproul, The Holiness of God, chapter 6.