I often heard atheists argued that even if a God exists, it does not mean it has to be a good or infinite or one God. They are implying that it is possible that there be an evil or finite or many gods. Are these reasonable assumptions or is it the case that God has to be necessarily good, infinite and one?

To understand what people might mean when they say that, if God exists, God need not necessarily be perfect, infinite, unitary, etc... it helps to consider which arguments in favor of God's existence are under discussion.

For example, consider the teleological argument which is, roughly, that the universe, or its creatures, have characteristics that could not have arisen through chance, or through evolution via natural selection. This argument sometimes takes the form of an analogy, and other times as an inference to the best explanation. In either case, though, the defender of this argument concludes, the universe and its creatures were most likely endowed with these characteristics by a creator. Given the nature of such a task of creation, this creator would need to be great indeed.

Now, this reason for believing in God -- because the universe and your own body, say, are too miraculous not to have a divine creator -- only requires a God capable of such creation. But such an act of creation could have been carried out by a committee of Gods, or by a massively advanced civilization, or an infinitely powerful supercomputer, and so on. So, if the theist believes in God for such reasons, then the atheist can say quite reasonably that such reasons don't require God to be one, or good, for example. After all, there is a lot of suffering in this universe, the atheist might point out, and if it we are inferring a designer or creator based on what we see around us, we might not think God is perfectly good or infinite, say.

Note here that it is not the atheist making assumptions so much as the theist about what kind of being God must be, in this case, capable of such an act of creation. The atheist then challenges the theist by noting that the reasons given in favor of God's existence may not require exactly the kind of God the theist had in mind. After all, showing that some being or other must have designed the human eye, or the laws of nature, does not help us to understand other features of this being (unity, goodness, etc).

Similarly, with other arguments for the existence of God, the theist can attempt to argue that a certain kind of God must exist, but, in each case, the argument will underdetermine God's nature. The Cosmological Argument, for example, requires only that God be the kind of being that could initiate a chain of causes that brings about the universe. An impressive feat, no doubt, but not automatically one that requires his goodness either.

So, in short, if the atheist is responding to a particular argument for the existence of God, then it seems reasonable to ask whether that argument really requires that God be omnipotent, omni-benevolent, unitary, infinite, and so on, because most arguments for theism do not require all of those properties without more argument. If, on the other hand, the atheist is merely asserting that, if God exists, God is as likely to be evil as good, or finite as infinite, or what have you, then it is the atheist who most likely would be expected to provide further argument.

A few links:
Entry on The Cosmological Argument at the Stanford Encyclopedia of Philosophy:
http://plato.stanford.edu/entries/cosmological-argument/
Entry on the Teleological Argument at the SEP:
http://plato.stanford.edu/entries/teleological-arguments/

Eugene Marshall's very helpful response explains that many different kinds of Gods, or even many Gods, might be compatible with the various different arguments for God's existence. I'd like to add just a minor, other point. If you take the Hebrew bible (or "old testament") very seriously, you might think there is also a biblical basis for rejecting the idea that God is wholly good, and even (depending on what parts you take seriously) that there is more than one god. So, some people might even think that there is a biblical basis for some of these accounts of God(s). Howard Wettstein has a nice essay on the former (I mean on God's not being wholly good), entitled "God's Struggles." Jeanine Diller also has some stuff on this. As for the idea that multiple Gods can be found in the Hebrew Bible, I think that's more of a stretch but I've heard some atheists appeal to that interpretative possibility.

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