Joshua uses the term God of your fathers
to draw attention to the fact that the Lord whom they serve is the one who revealed himself to Moses (Exodus 3:3, Exodus 3:15–16); it is God who made promises to the patriarchs about this land (Genesis 12:7; Genesis 15:18–21; Genesis 26:3–4; Genesis 28:4, Genesis 28:13; Genesis 35:12).
3 So Joshua said to the people of Israel, “How long will you put off going in to take possession of the land, which the LORD, the God of your fathers, has given you?