One could discuss without end the many difficulties of accepting the occurrence of a miracle. What evidence could possibly be convincing enough? What evidence should be admissible?
But in my opinion the most interesting question of all is what the occurrence of a miracle says about God's character. The most difficult problem with miracles is a moral one. Why should God intervene sometimes and not others? Why should the miraculous be rare? Why shouldn't it simply be a law that nothing bad can ever happen, that only justice will prevail because God will always intervene to see it done? This is the problem of evil.
On the other hand, why should God intervene at all? Why not rather leave nature as a closed system, with all creatures free to determine their own destiny insofar as they are able? To intervene is to show favoritism, which would be evidence of God's injustice.
There are various Christian answers to these questions, and what's interesting is to see how they line up on a spectrum between "grace" and "free will." Some traditions attempt to diminish the distinction between natural occurrences and miracles by asserting that everything is due to God's providence--all is grace. Others emphasize that evil exists in nature due to free will, and miracles are ways of God intervening to overcome the fall.
My goal is not to assess the strengths and weaknesses of each side, but rather to observe that each has chosen one side of a divide in our very conception of justice. At some point we have to be honest: no matter what God does or doesn't do--whether or not God does or does not exist--we will have some complaint to make against Him.
Which sort of universe would I prefer to live in? On the one hand, I could live in one in which God's providence covers absolutely everything, so that no evil could ever take place, because He would intervene to prevent it. In such a world, would my decisions be meaningful in any way? And how can I truly be a sentient being without any connection between what I decide and what then happens? In such a world, there would be nothing that could appropriately be called physical laws, because cause and effect would have to be entirely overridden by the supreme will of God who would prevent anything unjust. There would be no point in any sort of empirical science. By the same reasoning, I wouldn't see any utility in any sort of learning. What is the point of knowledge if the thing known has absolutely no connection with my ability to act in relation to it? My free will is intimately connected with my understanding of the world. It is only through conscious experience, in which I try one thing to see its causal connection with something else, that I can gain wisdom. This is so whether the experience is physical (as in a scientific experiment) or intellectual (as in a thought experiment). But if there are no consequences which cannot be overridden, neither is there any knowledge to be gained.
On the other hand, I could live in a universe, as many believe we do, which is entirely governed by laws or natural regularities and in which miracles do not happen. There is no reason why only atheists should accept this view; it is also perfectly compatible with deism. Why shouldn't it vindicate God's justice, after all, to think that nature is wholly bound by abstract laws? It is merely the classical theological principle that God shows no partiality taken to the extreme. More than the fact that all creatures are equally subject to nature's laws, we sentient beings are also thereby liberated. For if God does not intervene, then the only will that can be imposed on our surroundings is our own. By learning nature's laws, we come to master nature. By increasing our understanding, we increase our freedom. Is this not the grand aspiration of modern human beings? And if nature with all of its laws is the creation of God, should we not be thankful that He does not intervene? For if He did, then it would be our freedom against His; but if His freedom is bound by the natural order He has imposed, then our freedom is only limited by whatever is consistent with nature.
If miracles do occur in this world, perhaps it is in part to remind us that both of the views are insufficient. On the one hand, God's intervention means that our will can never be supreme. No matter how much we learn about the universe, we can never master it, for there is always a will superior to our own governing its very existence. On the other hand, the fact that such intervention is intermittent implies that we are free to a large degree, that our actions are not meaningless and that we ought to learn something about the abstract laws which create order in nature. Our science is essential, even if it is not all-encompassing.
Or perhaps the purpose of miracles is instead of draw our attention to laws whose "enforcement" is delayed for some reason. The miracle valued most highly by Christians is the resurrection of Jesus Christ, which previews the resurrection and final judgment of all people. Thus God's special intervention into human history serves (from a Christian point of view) only to confirm His impartiality.
But neither of these explanations resolve the essential tension between freedom and grace. The important fact is that such tension exists within the very concept of justice. Is the ideal society one in which everyone is provided for, so that nothing bad ever happens? Or is it one in which people are free to discover and invent, to do what is admirable or perhaps what is detestable? We have never been able to resolve this tension in our political life as human beings; I think this is related to our incapacity to resolve the theological question I have asked. Even in a somehow "perfect" world, in which the interventionist state could carry out its mission flawlessly or, alternatively, the state would be utterly consistent in enforcing the law--even then it would be difficult to see whether an interventionist or a liberal state would be ideal. So it is as least as difficult to understand God's own position as ruler over nature.
Continuing in my own Florenskian mission to allow no tension a resolution, I propose that once again the best, most righteous belief is to somehow believe in both grace and freedom--a leap of faith, to be sure. We must accept this world as a gift, and everything that happens in it is the providence of God. And yet we may contribute to it with our own minds, increasing in understanding and mastering it to our benefit and the benefit of others (and of the world itself). Nature is ordered according to fixed laws, yet it is bursting with God's purpose. How we embrace both must remain a mystery to the rational mind.
But in spite of this statement of faith, I will confess my own personal bias toward freedom, more specifically toward deism. I was born with a temperament such that I would like to believe all knowledge can be mastered, giving rise to ultimate freedom. This explains perfectly, I think, my attraction to mathematics and physics. But if the universe is to be truly transparent to the rational mind, it must be a closed system; there can be no room for miracles. Combined with the fact that I, personally, have never witnessed anything that could rightly be called a miracle, this has been a recipe for theological struggle most of my life. If I believe in Jesus Christ, I cannot be a mere deist.
Then again, the deepest places of my heart certainly recognize the allure of grace. In a way, one must understand the joy of not understanding. There is a suprarational element to faith that is far better expressed by poets or artists than by me. Yet even I can appreciate it in the depths of my soul.
As for whether any particular miracle has actually occurred, I still remain skeptical in most cases, because in most cases it seems not much is at stake. The one miracle on which my Christian commitment hangs is the resurrection of Christ, and for that I am thankful that there are people who have written so extensively about it. But I would love to remain open to the many other miracles reported by faithful Christians. It is not as if God's grace should put an end to my rational investigation of the world. On the contrary, it gives it meaning and hope.