By faith we make Him Lord of our lives. It is our faith that overcomes the world. When Satan can get us to doubt God or question our faith we are wide open to attack. Another possibility is that our effectiveness is hindered. Without faith we can accomplish nothing.
Faith is supernatural.
We do not depend on ourselves but on God. The Holy Spirit can help us to have faith in our time of need. It is important to understand that faith is also a choice that we have to make. It is an act of the will. If we choose to believe God He will honor our faith and strengthen it.