Every few months, a voice from the RSS says that they thing of all Indians as Hindu. This is what passes for a "moderate" statement among BJP supporters. So I want to break it down a little and provide some context from history and mythology.
The reason they "consider" all Indians as Hindu is because they think we all come from the same stock (like cattle) and that we are biological descendants of the same ancestors. It's just that our great and unbroken chain of kings and rulers was interrupted by invaders from outside (Muslims) and they converted a lot of "Hindus" to Islam.
This is the stated reasoning behind movements like Ghar Wapsi - "return home" (to your religion). Because the RSS thinks everyone should accept that they are of the same stock. Those who don't and decide to stick to the "invaders' religion" are therefore traitors.
When the RSS says it thinks of all Indians as Hindus, what you need to understand is that for them, there is no difference between the two. And this is why they can't think of Muslims as Indian.
Look at the framing - Muslims have a choice, but the RSS doesn't. Muslims can choose to abandon their religion, but the RSS can't abandon its idea of Hindu-ness. The Indian Muslim's condition is an outward affectation that can be dropped, but Hindutva stems from the fact that we're all descended from the same ancestors.
Do the gods teach of pure blood?
And this is where we run into a problem. Being descended from someone has never been an integral part of anything in Hindu mythology. Blood relations have always been secondary to other bonds.
None of the Pandavas were biological sons of Pandu, the king whose name they carried their entire lives. Ram and Lakshman were not born to the same mother.
The Ramayana pretty much starts with mention of Dashrath's inability to have children and also makes clear that paternity was not a matter of biology in that era. Krishna and Balram also were born to different mothers. Heck, even Pandu and Dhritarashtra were not biological sons of their father. Examples abound.
And guess who WAS related by blood? Vibheeshan and Ravana. And we all know how that went.
We are all "impure"
This obsession with blood and "stock" seems decidedly un-Indian. The pillars that have held India up since times immemorial have always been the ability to look at someone and say "let's hang out". Not obsessing over who is descended from whom. That smells distinctly foreign.
The trouble with making religious profiling a standard tool of propaganda is that in a country as diverse as ours, nobody is "pure" anything. There are no pure Hindus, no pure Muslims, no pure Christians. Purity is a toxic myth. And if you throw your lot in with those who make it the foundation of governance, policy-making, and legal protection on, sooner or later, your own "impurities" will come into play.
Whatever rules our religions had once upon a time to enforce purity, those rules have been broken over and over and over again. Those rules are dust. The quest for religious purity or blood purity (Hindutva style) is only ever going to lead to one destination. And that destination is a fascist future. A place where your children will always have to be on guard lest someone question their purity or have them declared less deserving of state benefaction because they are tainted somehow. Focus on pure hearts and human values. They're all that matters.