Since there is no clergy in Hinduism, and therefore no conversion rites, the general view is that while you may follow Hinduism, you can only be born a Hindu. A distinction is drawn between being born a Hindu and accepting /following Hinduism.
Yeah, but without there being a clergy, there is nobody to enforce this requirement on anybody.
There are several cases of Hindus who were not born Hindu, who have attained the highest posts in Hindu society. Hindusm, being a cosmological religion, does not contain any limitations based on time or place. In fact, in Hinduism it is presupposed that everybody is really Hindu, but they have degrees of amnesia. When one evolves they automatically become more and more Hindu. This is why I said that to gauge the spiritual development of somebody, you simply need to expose them to Vedic teachings, and see how well they resonate.
Erwin Schrodinger, Arthur Schopenhauer became Hindu simply by being exposed to Vedic teachings and resonating with them. Many of my friends, who are more spiritually developed than the average person, automatically resonate with Vedic teachings.
I have spoken to strangers before and floated Vedic teachings by them, and immediately noted how they responded to them. Some were not interested at all, but some were very keen and had a glint in their eye as if they listening to something that they always knew was true unconsciously. So to an extent it is true, they were born more Hindu. They are more spiritually developed souls.
Hindus cannot be humble on the matter of truth of religion. Our religion is true and all other religions are false.