The heap of ruins proclaimed God's judgment on sin. He took the sins of the Canaanites seriously, and could no more tolerate them; thus, he punished them with the destruction of the city. And he conquered the land for his people; thus, by Jericho’s ruins, Israel would remember why they were there and how they got there.
That also means that the ruins proclaimed God’s grace. No Israelite could pass by Jericho without seeing the writing of God, inscribed in the rubble: “You received this city out of my own hand as a gift of my grace, and through faith.”1
See Hebrews 11:30, which says that it was by faith that the walls of Jericho fell! So our gracious Lord appointed the ruins of Jericho to proclaim the gospel that God’s promises are received only as gifts of grace through faith!
34 In his days Hiel of Bethel built Jericho. He laid its foundation at the cost of Abiram his firstborn, and set up its gates at the cost of his youngest son Segub, according to the word of the LORD, which he spoke by Joshua the son of Nun.