Adi Shankaracharya was an 8th century philosopher and theologian who lived in India. He is credited with unifying and establishing the main currents of thought in Hinduism as well as consolidating the doctrine of Advaita Vedanta.
He is called a जगद्गुरुः (Jagadguru) meaning, "Guru to the world". Shankaracharya is widely considered to be the greatest philosopher of Hinduism.
His major works was composing various hymns, writing commentaries to Upanishads, Bhagavad Gita, Vishnu Sahasranamam, Brahma Sutra, Sanat Sujatiya etc.
Shankaracharya composed his literary works in Sanskrit and most of them were embedded with life lessons and philosophy.
Shankara lived in a time when foreign religions like Islam and new ideas like Buddhism and Jainism were competing for members. Shankara and his disciples acquired members for their traditions and debated with various scholars of different schools of thought, defeating them in the philosophical debates and establishing a firm hold of Advaita Vedanta.
Shankaracharya wrote over 300 literary works in praise of different deities and therefore gave Sanatana Dharma a new breath of life.
He established four monasteries known as the Chatyramnaya Peethas along the boundaries of India - at Sringeri, Dwaraka, Badrinath and Puri for propagating Sanatana Dharma and Advaita Vedanta. The monasteries still function as per Shankara's directions.
He introduced the Panchayatana Pooja, worship of five deities simultaneously. He is also considered to be the founder of Dasanami Sampradaya of Hindu Monasticism and Shanmata of Smarta Tradition.
Thus, Shankara saved Hinduism or Sanatana Dharma by devoting his entire life for study and spreading of his teachings and doctrines of Hinduism.