A witness Between Us that the Lord is God

Joshua 22:1-34
But be very careful to keep the commandment and the law that Moses the servant of the Lord gave you:1) to love the Lord your God, 2) to  walk in obedience to him,3) to keep his commands, 4) to hold fast to him and to serve him with all your heart and with all your soul.
Built a altar is to be a witness between us and you and the generations that follow, that we will worship the Lord at his sanctuary with our burnt offerings. Then in the future your descendants will not be able to say t outs, ' You have no share in the Lord.'
And the Reubenites and the Gadites gave the altar this name: A witness Between Us that the Lord is God.
Reuben, Gad and Manasseh have fulfilled their obligations to help in the conquest. Now they return home, with Joshua's blessing and a share of the spoil.
The fear that, once they are across the Jordan, Israel may at some future time disown them prompts the building of the altar which caused such misunderstanding. This was neither a sign of idolatry, nor a second sanctuary. It was a token of solidarity with the rest of Israel to whom they were bound by faith and worship of the one God.
Why?
Misunderstanding may cause war and damage. Israelite send some member to understand why they built a altar is good way to make clear between them.
Even there were witness, altar, a few years later, Israelite forgot the Lord their God.
How?
Remember the love and mercy in my heart, experience the love of the Lord then I won't forget the Lord.