The Book of Deuteronomy is believed to have been written by Moses as a collection of sermons delivered to the Israelites before they entered the Promised Land. It recounts God's faithfulness and commands obedience to His laws. Its purpose was to remind the new generation of God's power and instruct them to obey in order to receive blessings rather than curses. Key themes include obedience, blessing, warnings, and prophecies about a future prophet like Moses who would be the Messiah.