Psalm 91 is a psalm of trust and is especially meaningful to those who are exposed to danger and hardship. The words are a source of comfort and security and protection in times of sore need and deep distress.
Man's worship of God can never be correct until he has a right concept of God. Since God is invisible it is impossible to make images of Him. Even if we would never worship images it is still very possible to live in idolatry by worshiping a false concept of God.