Why is it that God commanded Joshua 3 times to be strong and courageous? Was Joshua stupid? Was he old in age? Or was God talking to a kid? None of the previous suggestions, of course. God knew and He knows that life here on earth is tough. He wasn't preparing Joshua for a battle but for many battles. Some people think that after coming to church life will become a bed of roses, that things will run as smooth as... the petals of those roses on the bed. Revelation 12.12 says all.
Earth is a battlefield. Our enemy, the devil, and his subordinates, the demons, are out and about devising plans and strategies to hamper our success. He plays dirty. He will attack from everywhere, at work, at school, at home, even in the church. It's our duty to be alert, always in spirit. The angels overcame the devil by the WORD of their testimony. Do I need to say more? It's war!