Perhaps no other word in the English language is as complicated as the word "god". The very word conjures up several meanings for people from different cultures and religious tendencies. The idea of god is perhaps one of the oldest in the history of human thought and with good reason - a great deal was unknown in those early days and it was rather convenient to have a one-size-fits-all kind of an answer to all the difficult questions.
If we look at the history of the word itself, we find that it comes from the Proto-Germanic guthan (similar to gott in German) which came from Proto-Indo-European ghut, "that which was invoked" (similar to huta in Sanskrit), the root word being gheu, "to invoke" or "to call". At any rate, there is no linguistic connection between the words 'god' and 'good' as is popularly believed.
Another related word is "divine," which comes from the Old French devin, "to conjure," "to guess," or "a soothsayer". And this comes from the Latin divinus, "of a god". The Latin word for god is deus, which comes from the Proto-Indo-European dewos (identical to the Old Persian daiva, Old Church Slavonic deivai, and Sanskrit deva), the root word being dyeu, "to shine" or "to gleam". This adds to the already plausible theory of the sun being the first God.
Now any religion worth its salt should have something to say about God. When we look at how the various religions visualise God, it becomes clear that there are vast differences (thus making it evident that platitudes like "All religions are the same" aren't worth even the paper they are printed on). At one end is Jainism, which is an atheistic religion and wholly denounces the existence of a Supreme Being. The foundational texts of Jainism state that the universe and all its constituents have always existed, and will continue to exist, so there is no need for a creator. Buddhism is also atheistic but Buddha does not deny the existence of God as much as he does not claim existence. He doesn't really care about God and creation but is more interested in how humans can avoid suffering here and now. The conception of God in many Eastern faiths like Taoism and Shintoism is comparable to Jain and Buddhist philosophy.
In the Abrahamic religions - Judaism, Christianity, Islam, and its various offshoots - the conception of God is somewhat alike (the differences in details make for an interesting study but are outside the scope of this article). Basically, there is one God who is all-powerful and all-knowing who created the universe and rules over it. God is an elderly male who has given certain rules for humans to follow, and a special man - the prophet - is chosen to share this with the world. There is little or no room for a different visualisation of God - for example, God cannot be a young girl.
Zoroastrianism too believes in a single God but uses a feminine term as well, perhaps to suggest that the Supreme is beyond gender. Sikhism goes a step further by claiming that there is only God and that divine being is timeless, formless and clearly transcends gender.
The conception of God in Hinduism is perhaps the most complex in all of the world's faiths simply because what we call as Hinduism today seems to be a conglomeration of several religious schools of thought. The fundamental principle seen in the Vedas, the Upanishads, and many later texts is brahman. Brahman is the Supreme spirit that is beyond creation and destruction. It is changeless, formless, and beyond thought. Since brahman pervades the universe, the universe itself is seen as God - there is nothing that is not divine.
While there is only one Supreme spirit, it takes on various names and forms. These manifestations of the Supreme - the various deities - are clearly human inventions. In one of the most beautiful poems in the Vedas (Rigveda Samhita 10.129.6) it says that the various deities are subsequent to creation. And who were these deities? Practically all forces of nature were worshipped as deities - sun, moon, sky, earth, wind, water, fire, space, rivers, mountains, trees, animals, dawn, and dusk, among others. Then there were other deities like Prajapati, Indra, Surya, Aditi, Mitra, Varuna, and Ushas. This was during the times of the Rigveda. Later on, Hinduism saw the creation of many new deities - both male and female. And the tradition continues as newer and newer deities are created until this day. For example, during the outbreak of plague in the early 20th century in Coimbatore, a new goddess was created - Plague Mariamman - who would protect people from the dreaded disease. In this easy manner, we create gods and we let go of older ones.
Only with this understanding of the multiple conceptions of God can we even begin an enquiry into the existence or non-existence of such an entity. Of course, there are many people who, like the Buddha, are unconcerned about this question in the first place. But for the others, like Richard Dawkins or the Pope, who are interested in this question, this broad understanding is imperative. Most of the so-called atheistic literature that comes from the West, often penned by brilliant minds like Dawkins, Hitchens, and Harris, seem to consistently miss out this broader view. Some of them are at least honest to admit that their critique is only aimed at the Semitic faiths but others tend to generalise religious beliefs using the yardstick of Judeo-Christian theology. This horribly fails when studying a religion like Jainism which is atheistic, or Hinduism which rarely clashes with science, or Buddhism which often speaks the language of the rationalist.
(In preparing this article, I have drawn from my discussions with Shatavadhani Dr R Ganesh and Dr Koti Sreekrishna.
For those of you interested in the science-religion debate, here's a recent discussion between two great scientists - Neil DeGrasse Tyson and Richard Dawkins)