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...