‘ And YHWH said to Joshua, “This day I will begin to magnify you in the sight of all Israel, that they may know that as I was with Moses, so I will be with you.” '
God now revealed to Joshua His purpose, that the people might realise that the God Who had revealed His power on Egypt was equally with Joshua. He knew how important it was at this critical stage that the people had an unrivalled leader in whom they could trust.
“That they may know that as I was with Moses, so I will be with you.” As the waters divided and they walked across on dry land, and remembered how their fathers had done it so long ago, they would be aware that here was another Moses who enjoyed the full backing of YHWH, and through whom YHWH would reveal His power.
Throughout the narrative we will be told that YHWH spoke to Joshua, but no hint is given as to how this took place, whether by commands within his mind impressed on him, or by a spoken voice. That it was YHWH Who spoke comes out in what results from obedience to His commands.