The qualities of an omnipotent god.
If god is almighty there are several qualities he must have. They are as follows:
He must know everything. Everything that is, everything that has been and everything that will be. To be able to know everything that will be he must know every position and every momentum of every particle in cosmos (Laplace's "World Spirit").
He must be worth our worship. A being that is not worth worshipping is no god.
He must be able to do anything. If there are things that god can't do, he certainly is not omnipotent.
He must be above time. Something that even St. Augustine deduced. But not only that, god must stand above all possible dimensions.
He cannot be 'good' or 'evil' or, indeed, have any subjective characteristics. If god is all good, he cannot do evil things and cannot be almighty. Most people would object and say that good can do evil but chooses not to do it. Well, if god is all good he can't choose to do evil things, can he?
The theodicé problem.
We also have the theodice problem, stated by David Hume:
If the evil in the world is intended by god he is not good. If it violates his intentions he is not almighty. God can't be both almighty and good. There are many objections to this, but none that holds since god is ultimately responsible for the existence of evil. Besides, if only god can create he must have created evil. If somebody else (the devil) created evil, how can one know that god, and not Satan created the universe?
Reasons not to believe in god:
The ontological evidence.
Necessary a god is a being that is worth worshipping, so if there is no being worth worshipping there cannot be a god.
Not any of the existing religions can provide such a god. How do we know if there are no undiscovered beings worthy our submission? Well if there is a being that has either failed or not tried to communicate with us that being is not worth worshipping either, so the ontological evidence against god holds, even without complete knowledge of the world.
There is a test, based on the ontological evidence against god, that you can do to try the existence of god. Pray, and ask god to provide you with a clear proof for his existence within a week. After that week, if you have got a proof that god exists, send me the evidence. If not, there are only three reasons I can think of that are plausible: (1) God does not exist, (2) God does not want to or (3) God can't give you this evidence. Because of the ontological evidence, alternative (2) and (3) are not worth your worship and thus they equal alternative (1). So if you get no response there is no god.
The meaning of the word existence.
What do we mean by existence? The very definition for existence is that a thing is said to exist if it relates in some way to some other thing. That is, things exist in relation to each other. For us, that means that something is part of our system ('The known world'). God is defined to be infinite, in which case it is not possible for there to be anything other than god because "infinite" is all-inclusive. But if there is nothing other than god then either god cannot be said to exist for the reason just explained, or god is the known world, in which case, by definition, god is not a god.
Occam's razor.
Occam's razor was formulated by William of Occam (1285-1349) and says: "Non est ponenda pluralites sive necessitate" or in english: "Do not multiply entities unless necessarily". It is a principle for scientific labour which means that one should use a simple explanation with a few explanatory premises before a more complex one.
Let's say that everything must be created, and that was done by an omnipotent god. A god which stands above time, space, moral and existence, which is self containing and in it self has it's own cause. This entity can surely be replaced by the known world. The world stands above time, space, moral, existence, is self containing and in it has it's own meaning. Most theists agree that god has a nature. Then we must raise the question, who created god's nature? If we just accept that god has a nature and exists without a cause, why not say that the known world just is and that the laws of physics are what they are, without a cause?
God is not really an explanation, only a non-explanation. It is impossible to gain information from non-information so God as an explanation is a dead end. When we have said that the reason for something is that 'god did it that way' there is no way to understand it any further. We just shrug our shoulders and accept things as they are. To explain the unknown by god is only to explain how it happened, not why. If we are to investigate the world and build our views of life from the world, we cannot assume a god. Because adding god as an explanation leaves as many, if not more questions than it explains, god has to be removed with Occam's razor if we are serious in investigating the world.
Some things are impossible to do.
There are things that are impossible to do. For example nobody can cover a two-dimensional surface with two-dimensional circles, without making them overlap. It is impossible to add the numbers two and two and get 666. You can not go back in time (without passing an infinite entropy barrier). The number of things that are impossible to do are almost infinite. If god were to be almighty he would be able to do them, but it's impossible to do so.
Some people say that he can only do things that are logically possible to do, but what is? Is it logically possible to walk on water? Is it logically possible to rise from the dead? Is it logically possible to stand above time, space and all other dimensions - and still exist? I'd say that everything which violates the laws of physics are logically impossible and thus omnipotence is logically impossible. Besides if omnipotence is a relative quality there is no way to tell omnipotence from non-omnipotence. For omnipotence to be a valid expression it must be absolute, but we have no objective criteria to measure omnipotence so the word itself is useless.
Omnipotence is impossible due to paradoxes.
Another way to disprove the almighty god is that omnipotence leads to paradoxes. Can god make a rock that is too heavy for him to carry? Can god build a wall that even he can't tear down?
Also, if god knows everything, he knows what he will do in the "future" (in any dimension, not necessary the time dimension). He must have known that from the very start of his own existence. Thus god's actions are predestined. God is tied by faith, he has no free will. If god has no free will god is not omnipotent. Another way to put it is that to be able to make plans and decisions one must act over time. If god stands above time he can not do that and has no free will. Indeed, if god stands above all dimensions god is dimensionless - a singularity, nothing, void!
Besides there can exist no free wills at all if god is almighty. If you had a free will, god wouldn't know what you would do tomorrow and wouldn't be omnipotent.
The void creator.
If everything must have been created, then god must have been created as well. If god is not created, then everything mustn't have a creator, so why should life or cosmos have one?
Besides this argument has another leap. If everything has a source and god is that source, then god must have existed without it before he created it. So if god created time and space, he must live outside of time and space. Thus he is non-existent. If all life must come from something and that is god, god is not alive and hence non-existent. If moral must come from god, god lacks moral. If logic comes from god, god is illogic. If nature comes from god, god is unnatural. If existence comes from god, god is non-existent. If god is the cause of everything, god is void
We would never notice god.
This is not an evidence against god, but rather describes the lack of sense in praying to a god who stands above time.
If god stands above time and created time and space he can not be the first link in a time dependent chain of events. Rather he would affect every step in all chains, and we would only see god in the laws of physics (Davies, 1983, chapter 4). This god is an unnecessary entity to describe the world and should be removed with Occam's razor
If somebody would pray to god and god would listen, the laws would change to achieve the desired result. Thus the world would be different and the prayer would never have been said. Besides god would already (in an "above time" sense of view) know that you would pray, and already have changed the world. Prayers would be totally meaningless. We would already live in the best world possible, and any prayer would be to doubt the wisdom of god.
Even worse: For every prayer said, god has not acted, or else the prayer had been undone. This means that the more people have prayed, the more bad things in the world have persisted. Therefore, the more you pray, the more evil persist (provided god exists and stands above time).
A much better way to change the world is to do it yourself. Then you would know that it was you who made the world better. The effect of prayers are not scientific provable, whilst the effect of actions are. Instead of praying you should set to work at improving your situation. This is what humanism is about.
Nobody really believes in god.