It is very difficult to explain to Abrahamic Religions to explain Dharmic Religions.
1 There is no single religious book of Hinduism. When someone says Geeta or the Vedas it is just one of the many books. You are free to follow any of these books or follow no book. There is no equivalent for a Quran or Bible in Hinduism
2 There are no set customs of Hinduism. A Gujrate Hindu will marry differently as compared to a Bengali Hindu, who will marry differently as compared to a UP Hindu.
3 There is no set "God" in Hinduism. When we say "33 Crore Devi, Devta" we actually mean it. You are free to worship any living/non Living - Being or Entity. You have Ram Worshippers you also have people who worship Ravan. He is considered a Great Gyaani Brahman and devotee of Lord Shiva
4 Like you correctly pointed out you can be an athiest and still be considered as a Hindu. You dont need to believe in god. I can go to a Church or a Mosque and pray there and still be a Hindu. Anyone who thinks and feels that he is a Hindu is a Hindu. It is not like Ahemadiya population. Even though Ahmediya consider themselves as Muslims the Fundamentalist Muslims dont do so
That is why saying Hinduism is a religion is wrong. It is a cultural way of life
There is a danger of believing your own propaganda.
1. Hinduism has Astika (Vedic) and Nastika (non Vedic) philosophies. If there is No book then there has to be a Guru to substitute the lack of a book.
2. There is no one custom in hinduism, but there is one core belief and that is "Dharma" and "karma".
3. "ravan worship" is a new concept and and offshoot of British inspired "Dravidian movement". Nobody really worships "talent", one worships the god consciousness like in Ram. One worships the adherence to dharma, not adharma.
4. Nastika philosophy like Charvaka or Ajvika also has the core philosophy of Dharma and karma.
Hinduism is a way of life, not only a cultural way of life, its also a social way of life, a value system, a dharmic way of life, a economic way of life etc.