Answer
Ancient age — Maharshi Vedavyasa
Maharshi Vedavyasa compiled, classified, and arranged the mantras of the Vedas. This was a monumental work of organizing the foundational scriptures of Hindu Dharma. He also composed the Mahabharata, thereby preserving the historical dimension of Indian dharma and culture. He compiled the eighteen Mahapuranas, gathering the diverse Vaishnava, Shaiva, Shakta, and other traditions into a single narrative heritage. In doing so, he gave ordinary people an accessible, story-based foundation in dharma. The Vedas and Upanishads were largely confined to scholars; the Puranas made dharma and adhyatma available to the common person in simple and engaging form.
Early medieval period — Adi Shankaracharya
Through his philosophical, educational, and organizational work, Adi Shankaracharya gave fresh strength to Hindu Dharma. He propagated Advaita Vedanta and established four major mathas in the four cardinal directions of India, thereby strengthening the cultural and spiritual unity of Bharat.
Late medieval period — Ramanandacharya
Ramanandacharya set in motion a broad and inclusive stream of bhakti. From his lineage emerged many great saints who carried the message of devotion to the common people, rising above the limitations of caste and birth. Through this, Hindu Dharma was given a new, inclusive direction.
Other great builders of the medieval period
Ramanujacharya, Madhvacharya, Nimbarkacharya, Vallabhacharya, and many other saints and acharyas made major contributions in the fields of bhakti, darshana, and religious education. Their efforts gave the bhakti movement its wide spread, and the various traditions of Hindu Dharma became even more deeply established.
Having understood the history and the human carriers of the tradition, we can now turn to its core teaching about life itself — what is the ultimate goal of being human?