First of all, I will equate "rationality" with "logic". At the very least, I will let illogic imply irrationality.
Explain why all religion is irrational.
Please bear with me, as it is a little difficult to get across the answer. Although it's easier to deal with each religion individually, I will attempt to assess a "general" religion.
Religion requires that someone (from now on "you") believes in a certain kind of supernatural power. Usually, religion brings up an issue of infinite importance for your belief and then provides some sort of proof, evidence or reasoning. For example, Christianity promises heaven to believers and eternal torment to unbelievers, the difference of which is infinite, and provides the Bible as the foundation of the belief.
Logically, decisions are made based on the information given and the importance of the decision, as well as the benefits and drawbacks. For example, someone telling me the story behind the story of Troy and the evidence behind it (assuming their story is the most likely explanation out of all given) might not have enough evidence, but seeing as it doesn't affect me too much whether I'm wrong or not, I'll believe it. However, someone telling me that if I jump off a cliff and can fly, providing the same amount of evidence/proof as in the other scenario, would not be enough to make me believe it, as the importance is much higher here: my life.
Unfortunately, all religion fails to provide sufficient proof, evidence or reasoning given the importance. Infinite importance requires absolute certainty of the religion's beliefs, which can never be provided using the proof given. That being said, I'm not even sure that one
can achieve absolute certainty about anything! The one thing notwithstanding is the fact that oneself exists, which sounds like logical reasoning and might well be construed with absolute certainty, but I'm not sure. However, anything else cannot be construed with absolute certainty.
This implies that all religious belief is illogical since it requires devotional belief in something that has infinite importance, while not providing the appropriately sufficient proof given the importance. And then we can apply the assumption above: illogical => irrational.
Why is all belief in god irrational?
The belief of a non-specific god or "deity-like" entity is a matter of personal explanation. If the universe makes more sense to you with god, then you believe in god. If the universe makes more sense to you without god, then you don't believe in god. If you're not sure which makes sense, then you might not choose to believe anything.
However, if we're talking about the belief of a specific God, then we run into problems. I shall define specific God as the exact opposite of what I just discussed above; basically you provide
some attribute or back story or any detail whatsoever on top of the aforementioned non-specific god. To believe that the specific god exists requires
some justification: there is some proof, evidence, or reasoning that this specific God exists.
Let's assign a value to the solidity of our proof. For example, if we saw "god exists" written on a piece of paper somewhere, we would define that as a "low" value. Whereas if we're using our best logical explanation, we'd probably value this as "high". However, we must accept our shortcomings: we couldn't possibly declare that our logical explanations are absolutely perfect, and that the logic system we're using is correct. I mean, after all, when we were smaller, we were sure that the best logical explanation for conception was the stork, as sure as we are now of our explanations. Thus, the value of our explanation is finite.*
We can also assign a value of importance to believing in this specific God. For example, heaven vs. hell would be a matter of infinite importance, whereas someday getting a truckload of donuts for believing we would call a "low" value. Now we can apply the logical decision-making model.
If the value of our explanation is greater than or equal to the value of importance, then we can believe comfortably. If the value of our explanation is less than the value of importance, then we're in trouble. We have something that's very important, but we don't have sufficient reasoning. Ideally, we will not believe until we can get enough reasoning to support this conclusion or find another conclusion that is better-suited.
Here is when things get unfortunate: we are not restricted to the belief of this specific God. We can conceive of other specific Gods, as well as non-existence of gods. The whole thing then becomes an exercise in futility. We have an infinite number of possibilities, each with their own value of likelihood, especially an unknown value of likelihood (since those "values" we were talking about can't be quantifiable). We can't know for sure that the value of reasoning of a specific God is actually higher than the value of another specific God, not to mention never being able to completely discount any possibilities. This effectively makes belief in a specific God logically have ?/INFINITY chance of being correct, where the ? represents the unknown strength of the reasoning behind the belief, and the infinity represents the inexhaustible supply of possibilities.
* - The validity of the logic in this argument is that it is the best system we have of evaluating things, so we will use it (or at least we think we have). Thus, the argument isn't moot due to the disregard of logic as ultimate authority.
I will now provide some examples of how I would logically make decisions on religious and supernatural beliefs.
Example 1:
I know the Christian God exists. I can always feel He is with me and I talk to Him on a regular basis.
But wait... could it be a delusion? If I were having a delusion, would I know that I am having a delusion? No. In fact, I can even recall back when I was younger, imagining that I had an imaginary friend to whom I talked on a regular basis. I can't know for sure that at any point,
anything (short of maybe my own existence) is not a delusion.
Also, how do I know it is the Christian God? What if this presence is actually a different kind of God that doesn't care if I regard Him as the Christian God? What if this presence actually has malicious intent? I can't know for sure.
However, I am
pretty damn sure that I am NOT having a delusion and this God that I feel with me is exactly who I think He is.
However, what is at stake? My eternal life?? So if I get this wrong, I suffer eternally??? Then I need to absolutely certain! 100%. I cannot take the chance that my holy book is wrong. I cannot even take the chance this is all a delusion! I can't believe something like this until I have attained the necessary amount of certainty.
Example 2:
I observed how the Christians have a holy book called the Bible and preach about God. Just because a book says so doesn't mean I should believe it, but the punishment for NOT believing this God is eternal torment. I would be a fool to NOT believe in their God. Thus, I should believe.
I observed how the Muslims have a holy book called the Qu'ran and preach about Allah. Just because a book... the punishment for NOT believing this Allah is eternal torment. I would be a fool to NOT believe in either God or Allah. Unfortunately, I can't be sure which is right, but I have to take a chance: that way at least I have a chance of avoiding eternal torment. As a rough estimation, I guess I have a 1/2 chance of getting it right.
I observe how I can conceive of a god that will punish me with eternal torment if I don't believe. Maybe given the evidence, God or Allah is more likely to exist, but who am I to say that a X-year old book is a better basis than my mind's conceptions? Then I observe that I can conceive of infinite such gods. My chances are now 1/INFINITY that I will not suffer eternal torment. Thus it is hopeless.
Alas, I now conceive of infinite gods who will not punish me eternally for lack of belief, and thus I have hope once again.
I could go on, but you probably understand the gist of my argument by now.
Holy crap, I've spent more than an hour writing this! It has been exhilarating, and it has helped me better form my own thoughts about this issue, having to pause and think many times, something clearing myself of misconceptions and better realizing what my beliefs really are.
Sorry for the long read, but a
general religious debate should be a long subject nonetheless.