God Wants You Well: What the Bible Really Says about Walking in Divine Health
Healing is in Christ's Atonement. Jesus died for our health just as much as He died for the forgiveness of our sins. This has to be the foundation of our faith for healing. If it is God's will to heal everyone, then why isn't everyone healed? It's not because of God. And it's not just because we don't have enough faith. We do have the faith to be healed. Jesus said the problem is our unbelief - unbelief cancels out faith. Instead of trying to get more faith or bigger faith, we just need a pure faith that isn't negated by unbelief.
There are laws that govern the spiritual world just as there are laws that govern the physical realm. Electricity has been around since the beginning of the world, but mankind didn't benefit from electricity until recently, not because it didn't work, but because of our ignorance of the laws of electricity. Likewise, God's healing is here and available. It's only our ignorance of the laws which control the flow of God's power that keeps us from benefiting from it.