(The view expressed in this blog are my own and should not be taken as inspired in any way.)
Joshua 3:7–13, “And the LORD said unto Joshua, This day will I begin to magnify thee in the sight of all Israel, that they may know that, as I was with Moses, so I will be with thee. And thou shalt command the priests that bear the ark of the covenant, saying, When ye are come to the brink of the water of Jordan, ye shall stand still in Jordan. And Joshua said unto the children of Israel, Come hither, and hear the words of the LORD your God. And Joshua said, Hereby ye shall know that the living God is among you, and that he will without fail drive out from before you the Canaanites, and the Hittites, and the Hivites, and the Perizzites, and the Girgashites, and the Amorites, and the Jebusites. Behold, the ark of the covenant of the Lord of all the earth passeth over before you into Jordan. Now therefore take you twelve men out of the tribes of Israel, out of every tribe a man. And it shall come to pass, as soon as the soles of the feet of the priests that bear the ark of the LORD, the Lord of all the earth, shall rest in the waters of Jordan, that the waters of Jordan shall be cut off from the waters that come down from above; and they shall stand upon an heap.”
There are four kinds of people on the earth: those who have been called by man, those who have been called by God, those who have been called by both God and man and those who have been called by neither God nor man. Joshua was called by both God and man. The Lord chose Him and Moses also chose him to be his assistant. (see Ex 24:13, “And Moses rose up, and his minister Joshua: and Moses went up into the mount of God.” and Num 27:22–23, “And Moses did as the LORD commanded him: and he took Joshua, and set him before Eleazar the priest, and before all the congregation: And he laid his hands upon him, and gave him a charge, as the LORD commanded by the hand of Moses.”) However, Joshua was also the newly appointed leader and still had to prove himself to the people. There are always some people who complain about the leader and suggest that someone else would make a better leader. Often these people suggest that the person complaining would make the best leader of all. The Lord understood these things and decided to show Israel that He had in fact chosen Joshua to be Israel’s leader. He told Joshua that He would be with Joshua in the same way that He had been with Moses. The was an important promise for the Lord had been with Moses all the time that Moses led Israel. Joshua could feel confident as he took charge of the nation and their armies. There was no one for him to blame any more if things went wrong. However, the Lord said that He would make it very clear to the entire nation that He was with Joshua. The people would see marvellous miracles and understand that the Lord was going to work in a miraculous way through Joshua. After all the miracles, there would be no doubt at all that the Lord was in charge and that Joshua was the Lord’s chosen leader. The River Jordan was in flood and there was neither bridge nor ford where Israel was going to cross but the Lord said that He would stop the flooded river and they would be able to cross over on dry land. So that people could remember what the Lord had done, Joshua was to choose a representative from each of the twelve tribes and ask them to select twelve stones from the bed of the dry river so that they could be set up as a memorial to what the Lord had done.