Jericho was the first city in Canaan, the land God promised to Israel. God said, "Jericho is My city -don't touch a thing in Jericho. The silver, the gold, the brass, and the iron are to be taken out of Jericho and put in the treasury house of the Lord" (see Joshua 6:19). Achan touched what God had set apart, and everything about him was destroyed. His wife, his family, his cattle, and his possessions were stoned and burned with fire.
Jericho belonged to God. The whole city was burned with fire, which speaks of judgment. Furthermore, Joshua warned that whoever rebuilt Jericho would pay with his firstborn! Israel could have any other city in Canaan. They were free to drive out the enemies and live there peacefully, which they did for forty years.
Do we realize what we do when we touch what is God's? This is a very solemn thing to consider. Notice that God told Joshua: "Up, sanctify the people; sanctify yourselves against tomorrow." This came after Israel's defeat at Ai. As the leader, Joshua was responsible to pray for his people and their sanctification.
When instruction from the Word of God is given and we say, "God, by Your mercy and grace I will do this," then the Word is sanctified by our obedience, and God's Word is honored as God's Word.