Typically cryptographers will define a security goal in rigorous terms, then prove that their system meets that goal (at least for "cryptomania" type applications like digital cash). There are good reasons for doing this:
1. It makes proofs of security possible, which make us a lot more confident about cryptosystems.
2. It allows us to be clear about what it means to "break" a system. If we are not clear about this, someone could claim that their system cannot be attacked by simply defining security to be the exact behavior of whatever they created. This is analogous to having a falsifiable hypothesis in a scientific experiment.
The original Bitcoin paper did not have such a definition. I am aware of one attempt at making such a definition, but it resulted in a very weak notion of security that placed unrealistic restrictions on what an attacker could do (basically, the authors were trying to find some definition that Bitcoin could satisfy; see point 2 above). In general, Bitcoin's security is highly suspicious, since by design the honest parties must scale their work in proportion with the work done by the attacker.
"Countries can be attacked too. I don't know what your point is here. If bitcoin survives its infancy, it could become more resilient than any other currency."
If you are admitting the possibility that an army might attack a country, then you are allowing Bitcoin to be fractured by an army destroying the outgoing Internet connections of a country. In the past, countries have been cut off from the Internet by accident (e.g. anchors being dropped on undersea cables). You could keep an attacking army outside of your territory and still wind up unable to communicate with the rest of the Bitcoin network. This is not a highly convincing argument.
Of course, this is all irrelevant, because a polynomial time attack is not the same as an act of war. There are a lot of organizations with the resources needed to perform the "51% attack" on Bitcoin and no compelling reason to think that a faster attack is not possible. You could attack Bitcoin by performing a lot of computation locally, without ever needing to step foot out the door. Bitcoin also does little to prevent attacks based on sending malicious messages into the network, despite the fact that cryptographers began developing techniques for dealing with that decades ago and despite the fact that almost all that work is freely available.
"3. Could you explain it in practical terms?"
Sure. Let's start with a thought experiment: I have something very rare, which has no practical uses but which is easy to give to others. Will you give me your car for a big pile of it?
Unless you are crazy you would not give up your car. The reason is that you are receiving something that is useless in exchange, and that you would have to go find some other person willing to take a pile of useless (but rare) items. Would your landlord accept some of these rare items as a rent payment? Would the bank accept some as loan repayment? Would the government accept it as a tax payment (let's just pretend that you are a law-abiding citizen who pays their taxes)?
How is Bitcoin any different? A lot of hype was generated about it, but at the end of the day you will not be able to pay your taxes with it, banks are unlikely to accept it for loan payments, and the majority of businesses that claim to accept Bitcoin payments actually accept payments in fiat currency via a Bitcoin exchange. Bitcoin currency has no practical uses (it is basically an energy sink) so the Austrian school does not support it, and it is not legal tender nor is it accepted for tax purposes by the government so modern monetary theory does not support it either.
"why should I care about what theorists say?"
For the same reason you should care about what cryptography theorists, physicists, and doctors have to say.
"we have never seen anything like Bitcoin taking off before."
That is because Bitcoin is the first of its kind. It is the first attempt to create a currency without a legal system. Even gold had value as currency because of a legal system.
"Yeah, I have seen a lot of people raging. How is that evidence of anything, other than fear of the unknown?"
You are calling the informed opinions of dozens of experts in cryptography and economics "raging" because they are saying that the system you love and support is based on dubious technical and economic ideas. It sounds more like you started out believing that Bitcoin is the future and are not willing to accept any argument that concludes anything else.