“Only be thou strong and very courageous, that thou mayest observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest prosper whithersoever thou goest.”
Joshua 1:7

God had told Joshua that he will not fail him nor forsake him, and he would be with him, just as he was with Moses. Knowing, that Joshua witnessed in close proximity how God worked with Moses, those statements must have been very comforting:
“There shall not any man be able to stand before thee all the days of thy life: as I was with Moses, so I will be with thee: I will not fail thee, nor forsake thee.” Joshua 1:5
Then God asked him to be strong and courageous, even if he had promised him that no man would be able to stand before him, he showed that Joshua would have to be strong and courageous to do God’s word, follow God’s instructions and rely on God absolutely in order for him to be successful in God’s purpose for his life:
“Be strong and of a good courage: for unto this people shalt thou divide for an inheritance the land, which I sware unto their fathers to give them.” Joshua 1:6
In your walk on the earth, strength and courage are necessary for you to insist on God’s word despite your optical observations, no wonder Paul prayed that the church be strengthened with might by the Holy Ghost- Ephesians 3:16.