1. The altar of Witness. The two-and-a-half tribes who lived on the other side of the Jordan decided to build an altar after they went back from helping the other nine-and-a-half tribes in conquering the land of Canaan. This became a controversy as the Israelites had set up the tabernacle in Shiloh as temporary centralized worship place for the Israelites (18.1). Hence to build another altar would symbolize a break from YHWH-ism and a split from the other Israelites.
The two-and-a-half tribes reassured their brothers and sisters that it was not what they meant to do. Indeed, they built the altar as a witness that they have taken their part in the collective effort to conquer Canaan, so that later on their descendants can still be allowed to worship in Shiloh (or Jerusalem, wherever the centralized location would be), since there is a possibility that the descendants of the nine-and-a-half tribes would object their rights to worship since they lived on the other side of the Jordan. One could forget history easily, and hence an altar was built to tackle this possibility. Appropriately, they named the altar Witness, since “it is a witness between us that YHWH is God.” (22.34)
And, by the way, when Jeroboam broke from Rehabeam, this is precisely what he did. He deliberately built alternative places of worship. He set up two golden calves in Dan and Bethel, the extremities of the northern kingdom (1 Kings 12). He knew exactly what he did, and that’s why he was notoriously known as the one “who made Israel to sin.” (e.g., 1 Kings 14.16)
2. The book of Joshua has been chastised for supposedly advocating holy war in the name of a god. Well, when you read through the book, it is equally, if not more, appropriate to say that actually what the book wanted to convey is the idea of a god who fought for his people: “For it is YHWH your God who has fought for you.” (23.3)
3. And a god who keeps his promises to his people: “Not one word has failed of all the good things that YHWH your God promised concerning you. All have come to pass for you; not one of them has failed.” (23.14) YHWH has kept his covenant; and now it’s the Israelites’ turn to keep theirs (23.15-16).
4. And one way to do that is to worship neither the gods of the Mesopotamia (where their forefathers came from) nor the gods of Egypt (where their immediate fathers came from). That’s how you should live in the middle of the Fertile Crescent (shown below). Neither right nor left; worship YHWH (24.15).
5. Incline, then, your heart to YHWH, God of Israel (24.23). The sentence perhaps sounds familiar to some of you. It is usually used in the end of church liturgy: “Condongkanlah hatimu kepada Tuhan.”
6. Finally, the theme of witness is reiterated at the very end of the book. Joshua took a large stone (by the Law he was not allowed to re-shape it) and set it up as a witness against everyone who had said that they would, indeed, serve YHWH and YHWH only (24.26-27).