Faith is the cornerstone of all Christians. It is how the disciples were able to preach and change the world. They absolutely believed in God's promises as told to them by Jesus Christ. The disciples treated the good news of Jesus as truth even though most of these truths were either promises for the future or at odds with the material world around them.
When Christians act out on their faith, it is the evidence of things not seen. Faith is the source of a Christian's strength and places them in God's plan for mankind. Through faith Christians anticipate the future of God's promises and act in faith in their day-to-day activity knowing they are in God's plan.
Without Christian faith there is no substance, no purpose, or strength in a person's life. People may have faith in other gods, themselves, or material things, but this type of faith is temporal and does not rely upon God's truth. Without faith in Jesus Christ, all is vanity with no purpose or hope. Jesus Christ came on this Earth to offer faith to all of mankind that everyone can be part of God's plan for eternal life and meaningful existence. Faith allows Christians to rise above earthly experience and participate in God's plan. See below for more commentary and Biblical advice on faith and Christianity.