“ Therefore be very courageous to keep and to do all that is written in the book of the law of Moses, that you turn not aside from it, to the right hand or to the left.”
Here was the condition, obedience to the law of Moses. It was to be strictly followed. They must be strong and have courage so that they do not deviate from it one way or another (see Joshua 1:7 and Deuteronomy 1:6-8. Joshua was steeped in Moses' teaching in Deuteronomy).