This is not a new question. The Internet Monk
recently asked similar questions. Many smarter and more knowledgeable people than I have proposed answers to this question, I'm sure. I just don't have time or resources to track down their answers. So I'm bound to be repeating things somebody else already said or making stupid objections to other views that the proponents have already answered. Oh well.
So, what do we make of this? According to the gospels Jesus healed lots of sick people; he gave his disciples authority to heal sick people. The gospel writers sometimes make a big deal of his willingness to heal the sick, including the disobedient (Mark 1:40-45) and ungrateful (story of the 10 lepers). So, what has happened to all the healings? Sure, Christians still testify to miraculous healings, but not on anywhere near the scale testified by the gospel of Mark, for example. You can find a pretty involved set of -- mostly dodges -- to that question here.
Look, either the physical healing ministry portrayed in the gospels is an integral part of the kingdom of God or it isn't. If it is integral to the kingdom of God and the gospels are telling us the complete truth about Jesus's ministry, we should be seeing the same mass healings today. This is essentially the argument of the Pentecostals. According to them, we aren't seeing mass healings today because of the massive unbelief of both church and culture. Medical science may actually be more of a hindrance than a help because it weans us away from reliance on God. If we weren't so dependent on medications, surgery, and other therapies, we'd see more direct, miraculous healings and have better health over the long run. As far as I'm concerned this view has been falsified beyond hope of rescue.
Let's say the physical healing ministry is not an integral part of the kingdom of God. Let's say instead that it was chiefly intended to establish the truth of the gospel. Once that is established (to God's satisfaction, I guess), miraculous healings recede in frequency, lest they become a distraction to the essence of the matter: repentance, faith, the forgiveness of sins, and the gift of eternal life. The close connection between physical healing and forgiveness of sins found in the early chapter of Mark is not meant as a paradigm for the immediate effects of entry into the kingdom. It serves as a sign to the new believer. Since his sins are forgiven, he has reason to hope for the elimination of all the consequences of sin, including any diseases, in the end. There are problems with this view. First, it brushes the compassion of Jesus under the rug. When he encountered a leper, he had compassion and "cleansed" him. Not only were the leper's sins forgiven, but his sickness was gone and with it all the social stigma he had borne. Today, a convert with bi-polar disorder has his sins forgiven and ... what? Does he get off the lithium? Usually, no. Where is the compassion of Jesus? About the only recourse left is modern medicine. Where did it come from, if not from the providence of God? I have a lot of sympathy with this approach, but it only sharpens the point of the question I raised in the title to this post.
If Jesus knew that a physical healing ministry of the type he exercised was not going to be a permanent part of the church's worldwide work, why did he not explicitly direct the disciples to begin developing its anticipated replacement? After all, modern medicine did not become a significant factor in the lives of most of the human race until the last 150 years or so. Almost nothing in the gospels or the rest of the New Testament suggests any effort on the part of Jesus or his early followers to develop any of the tools, techniques or underlying knowledge used by modern medicine to protect people from sickness or ease their suffering.
What does this mean?
1. Jesus and his first followers were profoundly ignorant of the natural causes and treatment of disease. They based healing on the direct, personal intervention of God and the faith of healer and healed, because they did not know of any other way to treat diseases more effectively.
2. Jesus did not want his disciples to learn about modern medicine. Why not? Maybe because it would become a distraction. But the miraculous healings were a distraction, according to John. And there are plenty of other central features of the kingdom that could become distractions. Take theology, for example. By the 400s that had become a huge distraction. Maybe, as the Pentecostals argue, it would tempt people to trust in man rather than God. But the persecution of the church tempted thousands to defect or compromise. Why didn't Jesus just eliminate the possibility of being persecuted? Maybe the early disciples were so woefully ignorant that even with direct teaching by Jesus they would not be able to maintain a responsible and effective research program for the long run. I suppose that would have been likely, if the initial results produced little benefit. But consider the good that could have been done with even the simplest advice and direction. Supposedly, these folks were committed to the divine authority of the Bible and took seriously its statements about things they would never see or hear even once in their entire lives. Could not Jesus have taught them about, say "unseen things" like bacteria, and how to avoid getting infected? Is this really more difficult for ignorant people to accept than the idea that God will raise the bodies of bad dead people to life and throw them into a lake of burning sulfur forever? I don't know what other possible explanations for Jesus's silence on this matter are left,
3. Jesus did not teach his disciples about modern medicine because he wanted unbelieving people to suffer sickness. The blessings of the kingdom ultimately are for believers. But it takes no faith to benefit from modern medicine. It is inherently subversive of priorities of the kingdom. Therefore, not only did Jesus not want his first followers to learn about modern medicine, he doesn't want modern Christians using it either. It is evil to resort to physicians, because the believing person does not need them and the non-believing person is wrongly protected from the consequences of his sins that would otherwise drive him to the Great Physician. I think this last explanation is a piece of pernicious bullshit. It also happens to be the kind of argument used by some theologians and preachers in past generations to stop the development of modern medicine. Don't believe me? Check out Chapter 13, section 10 of Andrew Dickson White's A History of the Warfare of Science with Theology in Christendom. Yes people complain that White tends to exaggerate the extent of the hostility, but the examples he gives are still instructive. Or, ask Turretinfan. He's the best at patristics I've seen on the web. If he can't find Christian fathers who questioned or outright condemned the pursuit of medical science, nobody can. He might even be happy to find them for you, if for no other reason than to show in what way(s) their questions/condemnations were correct. Mind you, most of the folks in his camp believe medical science is a good thing, and I would expect Turretinfan is no different. Its up to them to figure out how they can do this without adopting option #1 above, which I think is the only option besides Pentecostalism that does justice to the descriptions of Jesus as "compassionate."