If I am capable of grasping God objectively, I do not believe, but precisely because I cannot do this I must believe.
-- Søren Aabye Kierkegaard
When a friend ask me a while back: "Why can there be only one God?", I was taken aback a little. Sure, I am a Christian, believing in monotheism, but it had never crossed my mind why it is so. This was not ignorance but more like in my mind the inescapable conclusion is that God can only be one.
The argument I thus offered my friend was the causa prima argument:
Look at the world as a series of causes and effects with innumerable branches. An event is preceded by a cause, which is in turn preceded by another cause, and so on. Up the branches, we inevitably have to come to a point where there is a single cause that itself is not caused. This cause, or entity if you like, is called Causa Prima, the first cause.
To me, the extrapolation until every event is reduced to singularity of cause is inevitable. Animals cannot extrapolate far enough. An old dog-and-cat joke: If a dog is taken care of, it would deem the one taking care of him the Master. If a cat is taken care of, it would deem itself the Master.
Humans are then a little better. We see something greater than ourselves, then we extrapolate upwards to find God at the zenith of infinity. If we ever go along the way of arrogance of the cat, our rationale would tell us that a lot of things are beyond our control, therefore we ourselves cannot be gods.
By the way, the causa prima argument provides answer to chicken-and-egg question:
A chicken originates from an egg; an egg, a chicken; and so on. Sounds awfully familiar to cause-and-effect picture? This obviously has to stop somewhere up the origination branches. Simply: God creates the first pair of chickens. If you cringe at the word 'God', fine, you can replace the causa prima as anything else that causes the first pair of chickens. Evolution from another species, for instance. You should note however, that this causa prima is not the Causa Prima. It only is as far as existence of chickens is concerned. Regarding the existence of everything, subsuming chickens, the Causa Prima can only be a Supreme Being, with intelligence and consciousness.
You may realise that our extrapolation to find God is quite feeble. We have the concept of 'infinity' but it so taxes our mind when we try to rationalise it. The picture of God is then a fuzzy one, One whose personage we can only deduce from the ramifications down the branches of cause-and-effect. Religions then, can be seen as the attempt to illuminate on the identity and motives of this fuzzy God. This definition may not apply to some religions, like Buddhism where there is no God. Again, it depends on what religion means. Some people do not classify Buddhism as religion, but merely a way of living. If you look at the world's religions, monotheistic ones are surprisingly scarce: Zoroastrianism, Islam, Judaism and Christianity. Some might even group the latter three as Abrahamic religions collectively.
I should highlight that most religions view this attempt to uncover who God is anthropocentrally. Do good deeds, accumulate enough points to gain the entry to Salvation. In Christianity the picture is a little different: God understands that Man's picture of Him is fuzzy, so He went and revealed Himself, his personage, purposes, and ultimately His authorship of Salvation in Jesus Christ.
If you say doing good is enough for salvation, you have gone the path of arrogance of the cat. Who are we to say that our moral standard is good enough, that our 'good' is truly good? The Bible mentions several discrepancies between our own moral standard and that of God. If someone slap you in the cheek, you would be entitled to retaliate with another slap, an eye for an eye, right? No, Jesus said, give him your other cheek. You are entitled to love your friend and hate your enemy, right? No, Jesus said, love your enemy. That illustrates how Man finds himself deep in the mud of corruption, even his own moral is already corrupted; he cannot hoist himself to the higher ground. The only salvation is to reach the outstretched Hand coming down from above.
Doing good then is not the requirement of salvation, rather it is the consequence of it.
Fuzzy?