Perhaps we wish that God would perform a miracle for us that the whole world could witness on live television, or something? And this would clear up all doubt as to whether God exists. But He doesn't seem to do that. Rather, God performs miracles quite sparingly throughout history. There is a tendency to presume that miracles are an Old Testament thing, since we have the miracles of the Exodus, from the burning bush to the parting of the Red Sea and countless others in that whole story of Moses. And we have the great miracles of Elijah and Elisha, raising the dead and calling down fire from heaven. But miracles weren't actually very common in the Old Testament either. When the Israelites heard about Moses seeing a burning bush which did not burn up in the fire, they would have been as intrigued as we are. Is there some reason why God chooses to perform miracles at a certain time, and not at others?
The primary purpose of miracles appears to be God's way of authenticating a particular man as having authority given to him by God. Consider Moses, for example...
Then Moses answered, “But behold, they will not believe me or listen to my voice, for they will say, ‘The LORD did not appear to you.’” The LORD said to him, “What is that in your hand?” He said, “A staff.” And he said, “Throw it on the ground.” So he threw it on the ground, and it became a serpent, and Moses ran from it. But the LORD said to Moses, “Put out your hand and catch it by the tail” - so he put out his hand and caught it, and it became a staff in his hand - “that they may believe that the LORD, the God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you.” (Exodus 4:1-5)
The purpose of the miracle, and various others that Moses later performs in front of Israel and Pharaoh, is to authenticate Moses as the prophet of God and deliverer of Israel. Or we can think of other examples; Gideon asks God to perform a miracle concerning a fleece, so that “I shall know that you will save Israel by my hand” (Judges 6:37). When Elijah raised the woman's son she said, “Now I know that you are a man of God, and that the word of the Lord in your mouth is truth.” (1 Kings 17:24). When Elisha parts the river, the onlookers acknowledged that he had been given the same Spirit as Elijah. (2 Kings 2:15). Finally, we see that many of Jesus' miracles aimed to demonstrate that Jesus was a genuine prophet of God. In John it says 'When the people saw the sign that he had done, they said, “This is indeed the Prophet who is to come into the world!”' (John 6:14)
But when people saw Jesus' miracles, did they all believe that He was who He said He was; the Son of God? Many did not. Even miracles won't convince many people. If you want to know what I think it would be like if God performed a miracle on live television in this day and age, I think it would be something like when that alien autopsy film came out in the mid 1990's. It wasn't until 2006 that the film maker fully admitted that the film was actually of a fake model. But the point is that for all those years, most of the world remained unconvinced anyway. Not that they could prove it was a fake; but that even in the midst of what appears to be undeniable evidence of something extraordinary, people refuse to believe.
Whilst miracles are used by God to authenticate a prophet at a particular “landmark” in time, Jesus warns us to be wary of miracle workers...
For false christs and false prophets will arise and perform great signs and wonders, so as to lead astray, if possible, even the elect. (Matthew 24:24)
People who perform miracles are not necessarily genuine prophets of God, since there is real power in things like witchcraft. But miracle-workers can even think that they're doing these things as believers in Christ, yet be mistaken. Jesus said:
On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’ (Matthew 7:22-23)
What God wants is for us to believe in Him with a genuine faith. And as the Bible says, “We walk by faith, not by sight.” (2 Corinthians 5:7). This is the nature of faith; that it is trusting in God despite appearances. Now, many see this as believing in a God when there is no evidence. But that's not what I'm saying. We see abundant evidence for God, both in general revelation (the witness of creation all around us); the truth of God's Word, the Bible; and also in our personal experience of God. But Jesus said “An evil and adulterous generation seeks for a sign” (Matthew 16:4). Why? Because there is already sufficient evidence everywhere. When God said “You shall not put the Lord your God to the test”, this is not something we should find difficult to understand. It means that God will not submit to the will or commands of men. He’s already given us sufficient to believe in Him; and we know this is true because many millions of Christians do believe in Him.
In a sense, God is performing miracles all the time. God's sovereign hand guides all things. He “upholds the universe by the word of His power” (Hebrews 1:3). I often think that if it never rained, for example, then the first rain would be a miracle. Is it less of a miracle because it happens all the time, just as we wish miracles would? Oh, we can explain the causes of rain, but the whole water cycle is itself a miracle. God has created a world of order. A world in which miracles happened all the time, (in the sense of events which defy the natural order), then our idea of “the normal order of things” would be something quite different such that we’d be looking for something else as a sign; maybe for something with predictability? Really, we should see all of creation is a miracle, brought into being by the Word of God.
Blessed are those who have not seen and yet have believed. (John 20:29)