Joshua 3:7
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 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.
On that very day God would begin to publicly honor Joshua in the sight of all Israel. To magnify him meant to exalt him openly before the whole people, not in private but in their full view. The stated purpose was that they might know that God was with Joshua just as He had been with Moses. That promise gave Israel assurance of God's continued presence and protection, and evidence that He would work for them through Joshua as He had worked through Moses. By that evidence their confidence in Joshua's leadership was strengthened.
That strengthening was needed to prepare their hearts for entering the conquest of the land. It was a stupendous task, the same that had staggered the faith of their fathers forty years before. The promise was given before and as Israel prepared to cross the Jordan. At that time its spring flood made the usual crossings impossible.
The miraculous crossing fulfilled the promise and made the confirmation visible and public to the people. The passage through the flooded river confirmed Joshua's leadership and God's presence with him. The crossing displayed that presence accompanying Israel under Joshua's leadership, with Christ and angels going before the ark and the waters cut off before them. On that day God magnified Joshua in the sight of all Israel, and they feared him as they had feared Moses all the days of his life.