Paul responds to the charge that some might make (based on his teaching) that God is unjust. His response is brief and to the point, ‘May it never be!’.
Justice takes place when someone gets what he or she deserves. As sinners, we all deserve eternal punishment in Hell and it is just that we should receive this.
The question, then, is how can God still be a God of justice if He takes anyone into heaven? The answer is that Christ took their place and received the punishment they deserved (Hell) when He hung on the cross. So God’s justice is maintained but they are spared the punishment of Hell.
No sinner should ask God for justice - it is His mercy that we need!