A simpler way of providing evidence for God’s oneness is by referring to revelation. The argument here is that if God has announced himself to humanity and this revelation can be proven to be from Him, then what He mentions about himself is obviously true. The daring assumptions, from an agnostic perspective at least, are how do you know God has announced himself to mankind and in what form is this revelation?
Let’s take the last assumption first. If God has announced himself to mankind there are only two possible ways to find out: externally and internally. What I mean by “internally” here is that you can find out who God is solely by introspection and internalisation and what I mean by “externally” is that you can find out who God is via communication outside of yourself, in other words it is instantiated in the mind-independent world. Finding out about God internally is implausible for the following reasons,
1. Human beings are different. They have, what Psychologists call, “individual differences”, these individual differences include DNA, experiences, social context, intellectual and emotional capacities, gender differences, amongst many more. These differences play a role in your ability to internalise via introspection or intuition, therefore the results of introspection or relying on your intuition will differ. So you can see that if these processes were solely used to find out about God there would be inevitable differences in our conception of Him. This is true from a historical point of view, since the ancient world 6000 BCE, there are records of approximately 3,700 different names and concepts for God!
2. Since the method used to conclude that God does exist is a “common sense” method, or what philosophers call rational thought and w hat Muslim theologians may call innate thinking, then internally trying to find out about God would lead to fallacies. This is because what can be concluded using the universe as evidence for a transcendental independent cause is that it must be eternal, unique, powerful and personal; anything else would be speculation. The Quran aptly mentions “Why do you say about God of that which you have no knowledge?” If you try and internalise what God is would be equivalent of a mouse trying to conceptualise and think like an Elephant. It is obvious that the human being is not eternal, unique and powerful, therefore the human being could not conceptualise who God is. God would have to tell you via external revelation.
Take the following example into consideration, you know God exists like the knocking of the door, you safely assume that something is there, but do you know who it is? You weren’t expecting anyone, so you cry out “who is it?” in order to find out, and the only way to find out is if the person behind the door tells you. So you can conclude that if God has said or announced anything it must be external to the human being.
From an Islamic perspective this external communication is the Quran as it is the only text to claim to have come from God that fits the criteria for a divine text, these criteria include,
1. It must be consistent with the rational and intuitive conclusion on God. For example if a book says God is an Elephant with 40 arms you could safely assume that this book is not from God, as God must be external to the universe.
2. It must be internally and externally consistent. In other words if it says on page 20 that God is one and then on page 340 its says God is 3 that would be an internal inconsistency. Additionally if the book says that the universe is only 6,000 years old then that would be an external inconsistency as reality as we know it s that the universe is older the 6,000 years.
3. It must have signposts to transcendence. In simple terms it must have evidence to show that it is from God.
In the case of the Quran – and this post is not the place to discuss this in any depth – cannot be explained naturalistically therefore supernatural explanations are the best explanation. Some of these signposts include:
a. The linguistic and literary inimitability
b. There are historical accounts that could not have been known by man at the time of revelation
c. There are some descriptions of natural phenomena that could not have been known by man at the time of revelation
To conclude, since the only way to know what God has announced to mankind is via external revelation, and this revelation can be proven to be the Quran – then what it says about God is true. In the context of this discussion the Quran says “Know that your Lord God is one”.
These are some of the arguments that can be used to show that God is one; however this topic – once truly understood – will have some profound effects on the human conscious. The oneness of God is not only related to the fact that He is uniquely one, rather it refers to His worship, lordship, names and attributes, something that can only be tasted by pondering on reality, meditating on the meaning of the Quran, and becoming a manifestation of its message.
 IslamReligion.com: The Quran is the only unchanged Word of God. Previous revelations of God have either been lost or altered.