Be ye therefore very courageous to keep and to do, etc. - It requires no small courage to keep a sound creed in the midst of scoffers, and not less to maintain a godly practice among the profane and profligate.
That is written in the book - By the word of God alone his followers are bound. Nothing is to be received as an article of faith which God has not spoken.

Be ye therefore very courageous to keep and to do all that is written in the book of the law of Moses,.... To observe which required spiritual strength, fortitude of mind, a holy courage, a firm and fixed resolution, and particularly to destroy the Canaanites required in that book, and to abstain from all communion and fellowship with them:

that ye turn not aside therefrom, to the right hand or the left; See Gill on Jos 1:7.

BY PROMISES. (Jos 23:5-11)

the Lord your God, he shall expel them from before you, as the Lord your God hath promised you, &c.--The actual possessions which God had given were a pledge of the complete fulfilment of His promise in giving them the parts of the country still unconquered. But the accomplishment of the divine promise depended on their inviolable fidelity to God's law--on their keeping resolutely aloof from all familiar intercourse and intimate connections with the Canaanites, or in any way partaking of their idolatrous sins. In the event of their continuing in steadfast adherence to the cause of God, as happily distinguished the nation at that time, His blessing would secure them a course of brilliant and easy victories (Lev 26:7; Deu 28:7; Deu 32:30).