That is defined in the Quran and Hadiths. Those who understand them and really believe in them are true followers of Islam.
Who you call spineless cowards are those who have the real understanding of what Islam demands from them, and are willing and happy to give it. My thesis comes from this fact, and I am not stating this fact in order to support a premeditated thesis.
Yes, many religious Muslims will flat out deny that they are duty bound to strive against Kafirs, and that is precisely because they do not understand Islam. Islam does not give you the choice to pick and choose what you like and ignore the rest. The Quran is believed by Muslims to be directly dictated, word-for-word by Allah, the only true God, to Muhammad, the final messenger. If you will reject one thing, you have to reject the whole, for how can the supreme God of the universe be wrong about anything?
The Quran passages are not obscure - if anyone will say that they do not mean what they say, they are fooling only themselves.
As I have said before, many Muslims are content to just recite the Quran in Arabic without understanding it. Of course they are not bloodthirsty vampires. Many Muslims might be much better persons than many Hindus.
But those who understand Islam and believe what is teaches become the radicals and extremists.
