Let us understand HINDUISM first.
'Hinduism' is actually very-very little of a religion, if you talk of the essence of Hinduism.The more you call Hinduism, a religion, the more you are destroying the real thing. So, Hinduism is great only as long as it is not a religion. The moment you say, 'Hindu-ism is great', it's gone, finished.
Hinduism is not at all a religion in the sense Christianity and Islam are religions. The Upanishads do not propagate a religion at all. Are you getting it? How can you follow a religion after reading Ribhu Gita? Gone, finished! There is no question of religion. Yes, if this religiousness can be called 'Hinduism', then it is alright. Then it is alright.
So, there are religions who are on one plane, and then there is Sanaatan Dharma , which is another dimension. It is Sanaatan Dharma ; the eternal religiousness. It is not a religion. Sanaatan Dharma is awakened intelligence, also called 'Swadharma'. You know what you must do at any point, because you know who you are, that is Swadharma. Now, you don't need a dharma because you know Swadharma, that is Sanaatan dharma; Swadharma. That is what Krishna has talked about. When Krishna says, 'Swadharme nidhanamshreya (स्वधर्मे निधनं श्रेयः)', he is not talking about Hinduism. He is talking about the action of intelligence.
'Heretics', 'apostates', 'believers', 'non-believers', how does it matter? These terms are relevant only when there is a frozen pattern of thought, which you call as 'religion'. In the free and open expanse of mind, which you call as spirituality, there is no need for any religion.
When you have a religion then you can be converted to another religion, but once you start living in Swadharma, the Sanaatan Dharma , it is impossible to convert you.
So, who is a real Hindu? The one who is religionless. A real Hindu does not have any religion. To go beyond all religions is to be a Hindu.