The adjective adikos is rendered as unjust or unrighteous. There are two kinds of people, the just and the unjust, and God gives rain and sunshine to both (Matt. 5:45). The faith of Israel was fixed on this, that at the end of history there would be a resurrection of both kinds of men, the just and the unjust (Acts 24:15). At that resurrection, the Lord will bring the punishment He has been holding in reserve against the unjust (2 Pet. 2:9).The Pharisee was grateful to God that he was not like other men, who were unjust (Luke 18:11), but because he went home unjustified, he was in for a shock at the day of judgment when God would separate those who were truly just from those who were truly unjust.
God is qualified to render this judgment because He is ultimately and perfectly just Himself. Is God unrighteous if He judges unrighteousness, even if that unrighteousness makes Him look good by comparison (Rom. 3:5)? Of course not, and don’t be silly Paul might add. But if God forgot our labors for Him then He would in fact be unjust (Heb. 6:10) — but He will never forget our ministry of love to one another.
When Paul prohibits Christians from going to law with fellow Christians in front of unbelievers, he calls it going “to law before the unjust” (1 Cor. 6:1). If the unrighteous will not inherit the kingdom (6:9), then why do you let them settle your disputes? And besides, why would you want your case heard in a court that it run by the unjust? Or perhaps that is why you want it heard there?
Those who are unjust in petty transactions will also be unjust in weightier matters (Luke 16:10). Those petty transactions frequently involve unrighteous mammon (16:11), which means that Jesus is teaching us to be righteous in little things with petty unrighteous means, so that we will then receive true riches.
And last, Christ suffered on the cross in a substitutionary and vicarious way, the just instead of the unjust (1 Pet. 3:18). This is why there are two kinds of people in the world — the just and the unjust. If Christ had not died, the just for the unjust, there would be no just men at all.