The new Jewish Publication Society translation of the Tanakh renders Joel 3:21 as follows: "Thus will I treat as innocent their blood, which I have not treated as innocent; and the LORD shall dwell in Zion." According to this understanding of the verse, God will regard Israel as innocent, even though she is actually guilty. Meanwhile, God will destroy Israel's sinful enemies.
That doesn't exactly sound fair. Israel is sinful, yet God treats her as innocent. Israel's Gentile enemies are sinful, and God punishes them. Sounds like a double standard to me!
There are at least two ways to answer my qualm. First of all, one can argue that Israel did not get off scott free, for most of the prophetic writings are vivid descriptions of God's wrath upon Israel. That is true, but God definitely gives Israel preferential treatment. Jeremiah 30:11 says, for instance: "For I am with you, says the LORD, to save you; I will make an end of all the nations among which I scattered you, but of you I will not make an end. I will chastise you in just measure, and I will by no means leave you unpunished" (NRSV). While God destroys other nations for their sins, he merely chastises Israel. Granted, he makes her endure all sorts of hell, but he does not put an ultimate end to her. She's his chosen nation.
Second, one can say that God is fair because the nations are getting what they deserve. Just because God lets Israel off, that doesn't mean that he's treating the other nations unfairly. They are simply experiencing God's fair justice, whereas Israel is not. There is Scriptural support for this position, for God affirms in Exodus 33:19, "I will be gracious to whom I will be gracious, and will show mercy on whom I will show mercy." At the same time, there are other passages that condemn preferential treatment and uphold the same standard of justice for all, rich and poor (Exodus 23:2-3; Deuteronomy 16:19). And God asserts that he himself is impartial (Deuteronomy 10:17). In at least one strain of the biblical tradition, fairness means treating everyone according to the same standard, which excludes preferential treatment.
Calvinists and Arminians divide on this very issue. For Calvinists, God chose the people he wanted to save before the foundation of the world, while he condemned everyone else to their just punishment in hell. If you say to them that this is not fair because God is showing preferential treatment to one group and not another, they will inevitably respond, "God can save anyone he wants. He's not obligated to show mercy to anyone. We all deserve to go to hell. If God chooses to spare one group of people and not another, then that is his prerogative. It's his free gift to give." Arminians argue, by contrast, that salvation is available to everyone, but one must repent and believe in Jesus in order to receive it. In the Arminian scenario, God is impartial in that he offers to all people the opportunity to avoid hell. Those who choose not to receive God's gift will experience damnation, and they have only themselves to blame.
The Book of Joel has both Calvinist and Arminian elements. On the Arminian side, the Israelites must repent if they want God's wrath to cease. "Rend your hearts and not your clothing," God exhorts Israel. "Return to the LORD, your God, for he is gracious and merciful, slow to anger, and abounding in steadfast love, and relents from punishing" (Joel 2:13). On the other hand, God does appear to prefer Israel. In Joel, does God grant the other nations an opportunity to repent and avert their destruction? Not that I can see. In fact, God encourages the Gentile nations to fight so that he can demonstrate his strength and destroy them. Israel seems to have privileges that the other nations lack.