As a Christian who lives in a predominantly Hindu suburb I know from personal experience that it is important to understand Hinduism through the eyes of Hindus (and not just their Western interpreters).
So when a Hindu writes on “What makes a person a Hindu?” or “What does following Sanatana Dharma mean?” I pay attention. Here are some comments on just this from the Western Hindu:
“It seems to me that Shivheru is still following Sanatana Dharma, though by a non-orthodox path. Hinduism has always been open to allowing other influences, from Ramakrishna having images of Jesus and Mary on his altar to the Arya Samaj not allowing depictions of God. It is certainly possible to include Egyptian, Celtic, or other deities into the practice of Hinduism in exactly the same way that village deities can be added to the traditional Gods.”
“However, I don’t want to give the impression that anyone who includes some Hindu practices into their beliefs is a Hindu. I see a wide continuum from the orthodox Hindus to those which include aspects of other beliefs, and if someone on this continuum wants to call themselves a Hindu then I am happy with that. I am also happy if they want to say that they are influenced by Hinduism rather than being Hindus, the self-chosen title reflects a person’s spiritual outlook.”
“As soon as someone departs from the universal principles of Hinduism then I see them as no longer followers of Sanatana Dharma.”
This accords with my own understanding from face to face conversations with Hindus and reflections on the seven dimensions of religion. Hinduism is very open, but not so open as to allow anything. Hindus embrace many divinities. Nevertheless, they hold very strong views on karma, dharma and reincarnation. There are ways that are incompatible with being a Hindu.
Curious Christian 
I don’t totally agree with the quoted author. I appreciate that he allows for more breadth than many, but he simply doesn’t go far enough. Especially the notion of “universal principles” is very shaky. What are these? Where can they be found? “Hinduism” as a religion is a totally foreign and quite a young concept. Indeed there is not even a word for “religion” in the vernacular language of most Hindus. Even the appellation “sanatana dharma” is often completely unknown to very many Hindus.
It is much better to understand Hinduism in terms of culture and/or worldview and thus from a socio-anthropological perspective than from the viewpoint of comparative religion which is a Western construct that simply cannot adequately grasp Hinduism.
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