Hindu is the religion, Sanātana is an adjective

I see people, especially young people, proudly saying that they belong to the "Sanātana Dharma" or "Sanātana Religion", but I think it's not correct, at least grammatically. And in this post, I will try to explain what I mean:

First, let's understand the meaning of the word "Sanātana":

Sanātana (Sanskrit: सनातन) means eternal, timeless, ageless, and imperishable, referring to that which has no beginning or end. It represents absolute, unchanging, and primeval truths that are beyond time and space.

A fascinating word, isn't it? It truly is.

But let's now understand why it doesn't fit the context:

First, I will explain this with an example.

Jenny is a girl.

We do not have an adjective in this sentence, but we add one adjective, it becomes like this.

Jenny is a good girl.

Here, "good" is an adjective.

But what if I start calling Jenny as "good"? Hey good, how are you? Good, where are you going? How have you been good?

Weird, right? I mean, you can and should call her by name Jenny and not by an adjective.

Now, the same happens with in the context of Sanātana:

Hindu is a religion.

Hindu is a sanātana religion.

Here, Sanātana is an adjective. So... why would you say something like you belong to the Sanātana religion?

Obvisouly, I am not here to prevent anyone from saying what they want, but it was just a thought.