14 What shall we say then? Is there injustice on God's part? By no means! 15 For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.” 16 So then it depends not on human will or exertion,  but on God, who has mercy. 17 For the Scripture says to Pharaoh, “For this very purpose I have raised you up, that I might show my power in you, and that my name might be proclaimed in all the earth.” 18 So then he has mercy on whomever he wills, and he hardens whomever he wills. (Romans 9:13-18 (ESV)
Is there injustice on God’s part? Paul refuses to engage the question. God’s being is justice.
There is often a weird thing that happens with people. They often wonder why God allows so much evil in the world, but when God punishes evil they claim he is unloving. Perhaps we think it is justice when he is punishing others, but we cry foul when the whip is turned on us. The truth is, very few have ever thought through the consequences. Those who want to complain about God allowing evil would do well to consider the story of Nathan and David. Nathan tells David a nice story about injustice concerning a rich neighbor and a poor neighbor. The rich man steals from the poor man. When David pronounces judgment on the man with indignant anger, Nathan reveals that the rich man is David himself. We are all David in this story.
This isn’t to say that there isn’t a place for justice in this world, and for pressing for justice in this world. Temporal justice is one of the blessings God has bestowed upon us in this world. God checks evil in this world by using courts and governments to punish evil and reward good. This is never perfect in this world, and at times, these institutions themselves do evil and need to be called to justice. Yet, the world is much better with them than it would be without them.
When it comes to God, none of us are in a position to be crying for justice. What we can do is thank God for mercy. God is rather indiscriminate in whom he shows mercy. He will have mercy upon whom he will have mercy, and he will have compassion on whom he will have compassion. He will have compassion on you. He has had mercy upon you. This he did through his son Jesus Christ who died on the cross for your sins. As he says in 1 Tim. He desires that all men be saved and come to the knowledge of truth. This includes you. Justice was served on the cross that mercy would be served to you.