Miracles are Evidence Against the Existence of God.
Suppose that the miracle claims are true, and that Jesus, for example, did perform all of the miraculous feats that he is credited with. Typically, these miracles are taken as indicators that an omni-God exists. Here's the problem. Consider for a moment what sort of acts an omni-God would engage in. Being all powerful, all knowing, and all good, this sort of being will not act lightly. He would not make mistakes, he would not perform an act that did not accomplish exactly what he desires, in exactly the way he desires. He wouldn't be unclear about the outcomes of his actions in any way. So lets assume that if an omni-God were to act in the world, that action would be a perfect, flawless manifestation of that being's power, knowledge, and goodness.
Now consider some of the miraculous acts that are commonly attributed to God: Jesus is said to have walked on water, healed the sick, and resurrected the dead; Mohammed is said to have split the moon, and to have transcended directly into heaven, and so on.
This challenge has been put to theists concerning God's omnipotence: is God capable of acting in a way that would limit himself, such as by making himself not God, or making someone else God, or creating a challenge that he can't meet (like creating a stone that he cannot lift)? If he is, then there will be something he cannot do as a result of his action. If he is not capable of performing these kinds of actions, then, again, there is something he cannot do. So either way, God's power is limited and he is not omnipotent.
Theists like Aquinas and Plantinga have responded by pointing out that being omnipotent is having the power to do anything that is logically possible, or that does not involve a logical contradiction. All of these acts, they argue, are contradictory in some way. So these are impossible acts, and it is therefore no limitation on God's power to accept that he cannot do them. Thus it is widely accepted that the paradox associated with omnipotence conceived as the power to do anything is solved by understanding omnipotence as the power to do anything logically possible.
Now consider the purported miracles of Jesus and Mohammed above. Those acts were all minor, insignificant acts with regard to what an omni-God could do. That is, God is capable of doing far more than healing someone who is sick, or splitting the moon. He is alleged to have created the universe from nothing, after all. So it would appear that in those acts and all the purported miracles in history, God is acting far below his capacity. But it has been argued and widely accepted that an omni God wouldn't act in self-limiting ways. Doesn't that include acting in ways that are vastly beneath one's capacity? If I have a goal that I want to achieve, and I have means at my disposal to achieve it, it wouldn't make sense for me to only employ some of my abilities in a limited fashion to achieve that goal. I might act in a less than optimal way, applying some but not all of my knowledge, or some but not all of my power, if I don't understand all the relevant facts about the situation--I mistakenly think that the guy behind me in a marathon is too tired to catch me, so I don't push as hard as I could, but he's faking and he beats me, for instance. Or I lose the race simply because I don't have as much endurance as the next guy. Or I lose the race because I don't have the mental fortitude. But God won't have those limits in power, knowledge, or desire.
So it's hard to see why an omni God would act in such tiny ways. But it is easy to see why some lesser being, who is not God, might act in such ways. These miracles are the sorts of things that Vegas magicians would engage in. They are intended to impress by being flashy, provocative, and attention grabbing. These acts are localized, not universal the way an omni-God would act. These miracles are only seen by a handful of people (compared to the number of people that an omni-God could reach). These miracles leaves all sorts of doubts open and questions unanswered. In short, nothing about these acts suggest the infinite knowledge, power, and goodness of an omni-God. And everything about them suggests that someone of limited knowledge, questionable goals, and partial goodness like us was responsible. So it looks like that miracles, even if they were to happen, are actually evidence against the existence of God. The only sort of being that would perform such superficial party tricks is one who is limited in knowledge, power, and goodness. So there must not be a being who has unlimited knowledge, power, and goodness.