Summary: God's punishment upon Achan (and his family and possessions) seems severe...and it was, because sin is a serious matter. Sin is rebellion. Sin is covenant-breaking. Sin is choosing to go our way and reject God's way. And sin has consequences.
Hamlet famously complained that “Something is rotten in the state of Denmark.” Joshua found that something was rotten in Canaan, within his own people. After devoting himself to God, and after a miraculous victory at Jericho, Joshua's army suffered a defeat in Ai. Israel was enjoying what we'd call a “comfortable lead” only to have victory snatched away in a sudden reversal. Victory was so assured that the entire army of Israel was not required--utilizing economy of force, a principle of warfare...yet they were routed by the enemy and had no fortified cities to hide in! This was more than a set-back; God had assured Israel of victory. So what went wrong? The problem wasn't a superior enemy, but sin in the camp. Israel would've been routed regardless of their numbers.
When we're not getting the desired results, the problem may well be us. I've talked to people who've stopped coming to church, and often the reason is: one person in the congregation upset them. Just one. And they are only hurting themselves by staying away. But the damage is done. Let's not be the person who causes someone to stumble spiritually.
Jonathan Edwards is known for his fiery sermon, “Sinners in the Hands of an Angry God.” We don't like to think of God in such terms. He is a loving, merciful God--but He hates sin. Israel became what Achan stole--that which is destined for destruction. And they experienced God's anger burning aginst them.
The defeat at Ai was a shock, and it got Joshua's attention! Why didn't God tell Joshua immediately? To show how sin's consequences affect us all. Joshua felt personally responsible. He prayed in verses 7-9, yet his prayer sounds much like a complaint...which is OK; God can handle our frustrations. Joshua had a legitimate complaint as he was totally unaware of the cause of his defeat. God had promised victory. This was a shameful, dishonorable defeat that was liable to tarnish even God's reputation, His “name” (9) among the nations. These are words of despair, bewilderment, not unbelief. Complaining to God isn't the same as complaining about God. That's what Israel did in their wilderness wanderings.
We can complain, but we'd be better off to fear. Nothing could be more frightening than to hear God say (verse 12) “I will be with you no more, unless you destroy what is devoted to destruction.” “God is a consuming fire” (Hebrews 12:29), which means we need to take sin seriously.
Achan was the culprit; he was a soldier of Joshua's army; he was on the right side of the conflict, but he disobeyed the rules of engagement--namely, no looting. When we're not satisfied with what God has given us we're tempted to covet...maybe even to steal. Achan saw some of the forbidden spoil, was tempted, and kept it for himself. It should have gone into the Tabernacle treasury. The fact that Achan hid the plunder shows he knew he was in the wrong. Garrison Keillor noted, “Most people have done worse things in secret than anything anyone can falsely accuse them of.” The Epistle of James cautions, “desire gives birth to sin, which gives birth to death” (1:15). Achan had, “in effect, become a Canaanite by his actions” (Howard). And Israel became defiled. Are there times when we appear more like the unbelieving world in our actions?