First off I am a Hindu. I was born into one. I also love a nice prime rib medium rare and a hamburger once in a while. Now, do I fear that I would be excommunicated for that? Nope...cause there is no authority that can excommunicate me in the first place. I am very much a Hindu. I am free to believe what the Bhagwat Gita says or not believe a good portion of what the Gita says and I would still be a Hindu. I may even cherry pick what I want to believe from the Bible and the Koran. But does it mean there is no structure of any kind in Hinduism? Are Christians and Muslims Hindus?. Well let me bring a different context (Political) and see if that would help explain.
Context 1:
Western concept of liberty as a political system: e.g. "Life, liberty and the pursuit of happiness". It's a very broad framework. It does not go into the minutia or stipulate what it is that would make you happy etc.. Does it mean, that you can bring in a rigid system of communism or fascism ideology under that framework of "Life, liberty and the pursuit of happiness"? Obviously not since those political ideologies would violate the basic framework of liberty in the political context. However, you can still have socialism as normative values within the frame work of "Life, liberty and the pursuit of happiness".
Now apply the same concept when it comes to Hindu religion vs a vis other religions. There is no set ideology that Hindus should adhere t, to be a Hindu.. Hinduism is freedom of spiritual quest for an individual as long as the framework is not violated. if you insist that your belief or ideology is the only true one and the rest are bogus or that every other faith is in violation except yours, then you are violating the basic freedom of spiritual quest and most Hindus would not accept that as being Hindu.
Context 2:
Another illustration is how western liberal ideology is pooh poohed by some conservative society of the world by pointing out to the worst in western civilization as an excuse for they not adopting a free society. e.g. They often point to pornography in the west as failure of a free society "Life, liberty and the pursuit of happiness". They conveniently do not realize that westerners do not necessarily celebrate pornography instead consider that a price they have to pay for living in a free society.
Let me apply that to Hinduism. Because there is no rigid rules, you would find some odd and strange practices within Hinduism including some bizarre Tantric rituals. Missionaries and religious supremacists often illustrate these sects to point to the failure of Hinduism just like countries that have a tyrannical political system who point to pornography as the central tenet of free society. Most Hindus would tolerate these bizzare tantric practices (within the context of a law and order) and not necessarily celebrate them as Hindu customs.
Context 3.
Process of scientific quest: Scientists and the process of scientific quest is about the pursuit of that never reaching wall of knowledge. Its the pursuit and not about finding all the answers there is to know. Its about the constant debate. Yet, you would find some individuals (Creationists) who would use this as a weakness to deride scientists and what they do.
In Hinduism, its not about a set of revealed set of truth given to 1 or 2 individual that has adjudicated all questions and that subsequent generation would just have to accept this "adjudicated revealed truth" hence forth and that they would be punished if they challenge these "truths". Instead, Hindu beliefs are really musings of individuals (sears) over a period of time (over 4000 years) and these musings still continue and will never end just like scientific musings are a never ending pursuit.
So what does a free society believe? they believe in too many things just like Hindus.
That's my understanding of what is Hinduism. I tried my best to explain it. Hope my explanation was not too confusing.