How might the rights guaranteed in amendment i be stated as a basic constitutional principle? what basic principle does it protect?

Think about what you'd need for a democracy to work (or 'republic' if you prefer that term.) For The People to rule themselves instead of having an aristocratic class or warlords or dictators ruling them. What are the basic requirements? 

Well, first of all you'd need for people to know what was going on. You'd need good media to report the news truthfully and fairly ('fairly' in this case meaning giving both sides, or all sides). You'd need free speech, what's been called 'a free marketplace of ideas' so people could compare and choose ideas. You'd need freedom of assembly so people could get together to form parties or even to demonstrate and picket on the street to show their anger and outrage. You'd need to make it legal for people to write their representatives and tell them what they want without facing reprisals or arrest. Those are the bare minimum requirements for a democratic republic. 

Without them a democratic republic couldn't exist. In fact for 100 years there have been countries that CALLED themselves democratic republics but they weren't really because they didn't have these protections. Like Nazi Germany, the USSR, communist China, etc. 

One more thing. Our founders saw how Europeans had slaughtered each other for centuries over religion, so they wisely added that the govt. couldn't pick one religion to be 'official' and to give believers of that religion special rights and privileges, and also that it couldn't pass laws interfering with anyone's religion. So that's in the 1st Amendment too. Many genuine democracies do have an official religion, but they manage it in such a way that it's not a problem. 

So there are the 'five freedoms' of the 1st amendment. Speech, press, religion, assembly, petition