These comments help us to realize that Joshua is drawing attention specifically to the old covenant. The old covenant was the one made with Israel at Mount Sinai. This covenant governed Israel’s life in the land stipulating blessings for obedience and curses for disobedience. Thus if Israel turn from the Lord, they will receive curses. Their sins as a people will not be forgiven and Israel will be cast from the land. This (old) covenant, however, does not annul the former covenant which God made with Abraham (Genesis 12:1–20, Genesis 15:1–21, Genesis 17:1–27). Under that covenant (the covenant of grace), God will forgive the sins of his people if they believe his promises to save sinners (Genesis 15:6).1 Since the old covenant was an administration of this covenant of grace, any and all promises of sins being forgiven (e.g., Psalm 103:9–12) should be understood as being grounded in the promises made to Abraham and not the covenant made with Moses at Sinai.
19 But Joshua said to the people, “You are not able to serve the LORD, for he is a holy God. He is a jealous God; he will not forgive your transgressions or your sins.