Joshua 1:7 (CSB) "Above all, be strong and very courageous to observe carefully the whole instruction my servant Moses commanded you. Do not turn from it to the right or the left, so that you will have success wherever you go.

Success comes from God and doing what His word tells us to do. Moses had left to Joshua the first five books of the bible. That was all Joshua had to work with. Now we have 61 more books to follow. If all scripture is God inspired (2 Timothy 3:17) then we can trust that by living according to the whole bible we can have success. Joshua was to be strong and very courageous to carefully observe the whole instruction that Moses had left. He could not pick and choose which to follow. He had to follow the whole thing. We are also to follow the whole bible and not just the pieces we like. We are to love everyone from our family to an enemy, everyone! The bible helps keep us on the straight and narrow (Luke 13:24) as long as we don't stray from it to the right or left.