What do proponents of 'Hindutva' believe is necessary for building a strong nation?

A united national culture.

The term 'Hindutva,' coined by its originator V. D. Savarkar, literally signifies 'Hinduness.' In Savarkar's understanding, it forms the foundation of Indian (also Hindu, in his terminology) nationhood. Fundamentally, it proposes that to be integral members of the Indian nation, individuals must not only acknowledge India as their 'fatherland' (pitrubhu) but also as their sacred land (punyabhu). Advocates of 'Hindutva' contend that a robust nation can only emerge from a steadfast and unified national culture. They further assert that, in the context of India, this foundational element can solely be furnished by Hindu culture.