Why did God say He would magnify Joshua in the sight of all Israel?
God magnified Joshua publicly so all Israel would know He was with him just as He had been with Moses, confirming Joshua as His chosen leader to bring the nation into and occupy Canaan, land still inhabited by Canaanites and tribes God promised to drive out.
Les explains that in verse 7 God told Joshua, who had now taken Moses’ place, “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.” The purpose was to prove to the nation of Israel that Joshua was indeed now His man to lead them in and occupy the whole area of Canaan. That land was still inhabited by the Canaanites and the various tribes God had told Abraham would be driven out so that His offspring could have the land of Canaan. Les further notes that when Israel came to the brink of Jordan, they were to stand still, and as they stepped in, the water suddenly piled up. Like at the Red Sea, Israel crossed Jordan on dry ground even at flood stage. Les constantly reminds his classes that when you deal with God and Israel, the supernatural is commonplace. We should never be shocked by any supernatural miraculous event when God deals with Israel. In the age of grace, God has chosen not to perform the supernatural, though He remains the same sovereign, all-powerful God; the reason given is that He has given us His Word.
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.