"Now we know that God's judgment against those who do such things is based on truth"(Romans 2:2)
To be a judge in our court system several requirements have to be met. First the judge has to know the law. They have to compare the offense against the standard of the law and render a decision. Secondly, the judge has to be impartial. They cannot be bias towards either side of the case. And lastly, the judge has to be fair. They have to base their decision on the facts or the truth of the case. Opinion cannot play into part during the trial. Both sides have to be weighed equally and whatever side is proven right wins out in the end.
In the previous verse, Paul has just disqualified mankind from rendering judgment against each other. "You, therefore, have no excuse, you who pass judgment on someone else, for at whatever point you judge the other, you are condemning yourself, because you who pass judgment do the same things." ( Romans 2:1 ) To participate in the transgression excludes the ability to render fair and right judgment.
However, given the criteria for passing judgment, Paul knows that only God is capable of doing it. God's judgment is based on truth. Many people thing that truth is a relative term, but in reality truth is an absolute. And that truth is based on His Word and His character. Knowing from our own court system how important the truth is in deciding a verdict, one can rest assured that God bases everything upon it. If he didn't he would not be just.
God is just but God is also merciful. Paul tells us in Romans 3:26, "...so as to be just and the one who justifies those who have faith in Jesus." This is extremely important because as a just judge he has to render his decision based on truth, but as a merciful savior he took the punishment for us we could be justify before God. He render the decision of guilty and the penalty was death or eternal separation from God. But, when we believe in Jesus and accept His free gift of Salvation then we stand in true righteousness and holiness before God 'just as if we've never sinned'. The judgment and penalty was placed on Jesus and the benefits were given to us.
Praise Jesus that he is both a just and the justifier!