The primary purpose of miracles in the Bible was that of authenticating the messenger and the message. Casting out demons and healing people was Christ’s way of showing who He was. While He had great compassion on the people and certainly healed them out of His compassion and love, that was not the primary purpose of the miracles nor the primary purpose of His ministry. Christ came as man that He might die for our sin and bring spiritual healing to bring us back to God (see 1 Pet. 2:24-25). Note that the focus in this passage in 1 Peter is that we might die to sin (its reign) and live to righteousness. The healing in the cross Peter is talking about is spiritual and is specifically stated in connection with spiritual growth and change and being returned to the Shepherd and guardian of our souls. A good illustration of this point is found in Mark 1:32-39.
Yes, Jesus healed many and cast out demons but these were few in comparison to those He did not heal, Even though many were seeking healing from him, he journeyed on to other locations in order to preach the gospel because healing was not the primary reason He was here. The miracles authenticated and showed he had power over disease and demons and was thereby the only one who could truly save men from their sins.
The idea that all disease is the product of demonic activity is false and not supported in Scripture. We live in a fallen world where many things contribute to disease. In many ways this is the product of Satan’s activity and man’s sin, but often not directly. The healing Jesus and the apostles did was always distinguished from casting out demons and those diseases caused by Satan or demonic activity. They are not always one and the same. We too often want to blame everything on Satan when often the Lord wants us to realize the problem is us, or simply a trial He wants to use as a tool of growth or to demonstrate His strength through the trial. God is not a Genie that we can rub to get our wish and make life as we want it. It takes more faith to trust God through such trials (sicknesses, pain, suffering) than to simply experience immediate relief or the removal of the pain.
Can God heal us? Certainly, and we should pray for that if it is His will, but we must also seek His wisdom in finding other solutions (diet, medical help, etc.) and have the faith to rest in what He wants to do in and through us in the process. Above all, we know He wants to mature us spiritually regardless of whether He suddenly or gradually heals us or not. Our need is to seek help, pray for wisdom, removal if it’s His will, but above all to rest in His fatherly grace and plan.