Clearly the central theme of the book of Joshua is the truth that God is a promise keeper. Well over five hundred years before these recorded events God pledged to Abraham that 'to your offspring I will give this land'. The land he is referring to is Canaan. From that time onward to the period of the conquest God repeatedly makes the same promise to his people. This promise occurs so frequently in the Pentateuch that it seems to be the theme sentence of the literature. The writer of the book of Joshua recognizes the importance of this theme. The fulfillment of God's promises of a land are central to the book. He begins the book with it and brings to closure Joshua's final charge to Israel's leaders with it. And the promises of God as fulfilled are found throughout the book.
