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. Many Scriptures relate the healing of our bodies and the forgiveness of our sins in the same verse. Jesus went about healing all who were oppressed of the devil and told us to do the same. Jesus hasn't changed, it's the people who represent Him who have changed.
What about Paul's thorn in the flesh It is commonly believed that the apostle Paul had a sickness for which he besought the Lord for healing three times, and the Lord wouldn't heal him. From that assumption, people teach that God sometimes wants us sick. In this book, Andrew reveals what Paul's thorn in the flesh was, and it wasn't sickness.
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.