Torah Is Our Constitution and Our Body of Laws

An example:

American Citizenship. We (as a whole of American citizenry) have no problem accepting that a person born to American citizens is an American. We also accept that the constitution and the laws of the land define what it means to be a good citizen. And we accept that not just anyone can become a citizen. They need to live here, agree to obey the laws, learn about the US, pass a test, take an oath.

If you and I decided that someone from another country should be a citizen, we can't just hand them a flag and say they are now a citizen. Even though you and I might think that person is a wonderful person and will make incredible contributions to the USA, they cannot become a citizen outside of the law.

Similarly, in Judaism, Torah is our "constitution" and our body of law. The difference is that it was not authored by man, but by G-d. We do not have the right to change it. We DO have the right to apply its teachings according to a given set of rules.

There are lots of Jews who do not abide by the body of laws given by G-d but they are still considered "Jews," just as there are lots of Americans who do not abide by the laws of the land are still considered "Americans." 

Torah is not merely a document - it is a Living Revelation of the Living G-d.