Today's Gospel reading in the Ordinary Form of the Roman Rite is Mark 2:12: Jesus' healing of the paralytic brought to him by four men who, because of the crowd inside and around the door, cut a hole in the roof of Jesus' own home and lowered the poor fellow down. If it had been my home, my first thought would have been to look for the guy who should get the repair bill. But true to form, Jesus is not said to have cared about that. Nor does it even seem that his primary concern was the physical healing he did. His concern was with the forgiveness of sins. Addressing those who thought him blasphemous for presuming to forgive sins, he uttered: "But that you may know that the Son of Man has authority to forgive sins on earth," and then said to the paralytic: "Rise, pick up your mat, and go home." That suggests a connection on which we need to meditate if we are to answer the question "Why doesn't God heal all the faithful?"
Together, the four Gospels leave no doubt that it is faith in the Lord which heals people. Conversely, lack of faith in Him hinders healing (Mark 6:5). But whose faith? Not the Son of Man's faith, which is indeed necessary but also a given; the key, rather, is the faith of the believing community. Other Gospel stories, such as that of the healing of the centurion's servant (Matthew 8: 5-14), as well as the experience of the faithful through the centuries, indicate that the needed communal faith can be wholly vicarious: the one healed hasn't done or professed a thing to be healed, but rather benefits from the faith-filled intercession of others. Indeed, today's passage from Mark says that it was because of "their faith" that Jesus undertook to heal the paralytic. It isn't made clear whether the paralytic himself was included in "their," but that hardly matters. What seems to matter is the aura and energy of faith with which the person in need of healing is surrounded. That's why faith-communities in which belief in miracles is lively tend to get more miracles. Indeed, it can safely be said that the more faith a body of believers has, and the more that is manifested in loving intercession for each other, the more medically inexplicable healings will occur.
But that doesn't mean there's a law of nature we can derive accordingly. We cannot say: "Miraculous healings vary directly with the level of faith in the community." For one thing, neither variable of the equation is can be measured with the requisite precision; for another, God's sovereign freedom in manifesting his irrevocable grace cannot be circumscribed. In any event, grace in the primary sense is God's self-communication of his life to us, so that we may become "partakers of the divine nature" (2 Peter 1:4). So understood, grace is supernatural, "beyond" mere nature, and so does not work in quite the same way as those regularities which we call "laws of nature."
Nevertheless, there is a kind of predictable regularity to God's mercy. The divine life he won for us by his Pasch is always on offer, because it is offered to all and suffices for all. Divine mercy is simply divine grace as received by sinners; Jesus came to save "sinners" not the righteous, and "all have sinned and fallen short of the glory of God." So his mercy is manifest whenever we receive him in faith and repentance, as countless people have done and will do. Some experience it only vicariously, e.g., those among the baptized who die before reaching the age of reason. And as we have seen, miraculous physical healings testify to God's mercy in healing our souls, i.e. bestowing his transformative mercy on us. Yet most of the fervently faithful who are sick or disabled do not experience miraculous healing of their physical maladies. How can that be, if faith fosters miracles and only lack of faith hinders them?
Well, the kind of miracle we're talking about has to be relatively rare. The term 'miracle' is from the Latin for 'something to be marvelled at', and what is to be marvelled at here is precisely a striking departure from the laws of nature. That is only to be expected in a world in which grace supervenes on and elevates nature without replacing it. For if the workings of divine grace abolished the laws of nature, there would be no Nature. But whatever our ultimate end, we are now and necessarily part of Nature; so if Nature were forthwith abolished, there would nothing in us for grace to elevate.
Yet the Gospels make quite clear to us that miraculous healings are outward signs of God's mercy. As such, they not only depend on faith but are meant to elicit faith. They are by no means the most frequent occurrences in human life or even ecclesial life; rather, like the "seven" sacraments, they are communal signs that help to bring about the realities they signify. As shown already, one of the realities which miracles of healing, as "sacramental" realities, both signify and bring about is faith. Now faith is a virtue, an habitual response to God. But as such, it is also and necessarily a gift: a manifestation of grace in the fundamental sense, i.e. God's life in us making it possible for us to attain such deep intimacy with him that we can undergo theosis, divinization.
When that life is fully manifest in God's people on the Last Day, there will be no more physical maladies, indeed no more death. God will have healed all the faithful, who will have been liberated from "this body of death" (Romans 7:24) and given new, "spiritual" bodies (1 Corinthians 15:44). But on our earthly pilgrimage, we must learn above all to die to self and live to God. That is our baptismal vocation; that is our initiation into the very life of God, our apprenticeship as partakers of the divine nature. When accepted in loving surrender to God, the unavoidable physical realities of sickness and death are meant to help all die to self. Even when the one afflicted has never had the capacity or opportunity to commit sin, they are an invitation, a sign, to the rest of us of what each of us must do in faith and love. Yet, when sought and received in deep faith, miracles of physical healing manifest and help to bring about that life into which dying to self is the point of entry. They help to keep us faltering pilgrims in via ad patriam.