We built that altar because we were worried. Someday your descendants might tell our descendants, "The LORD made the Jordan River the boundary between us Israelites and you people of Reuben and Gad. The LORD is Israel's God, but you're not part of Israel, so you can't take part in worshiping the LORD." Your descendants might say that and try to make our descendants stop worshiping and obeying the LORD.
To build another altar for offering sacrifices would be the same as turning our backs on the LORD and rebelling against him. We could never do that! No, we built the altar to remind us and you and the generations to come that we will worship the LORD. And so we will keep bringing our sacrifices to the LORD's altar, there in front of his sacred tent. Now your descendants will never be able to say to our descendants, "You can't worship the LORD." But if they do say this, our descendants can answer back, "Look at this altar our ancestors built! It's like the LORD's altar, but it isn't for offering sacrifices. It's here to remind us and you that we belong to the LORD, just as much as you do."
Please listen to what I have to say. If the LORD has turned you against me, maybe a sacrifice will make him change his mind. But if some people have turned you against me, I hope the LORD will punish them! They have forced me to leave the land that belongs to the LORD and have told me to worship foreign gods.