Buddha introduced organized monasticism in India. I would not use the word organization in connection with the ancient rsis. It was left to Buddha to organize monasticism as an institution; he was the first person in the history of religion to establish monasteries and lay down rules of monastic living. It was one thing for a wandering monk to go from place to place; the rules by which he was governed were the rules of his inner life. But when you have one hundred monks living together, then other rules have to be instituted so that they can live together in peace and with profit. Buddha was the first to institute such principles of discipline. All these things he did.
After Buddha there came a long period during which there were many teachers, but none were of such prominence, and I won’t go into details. Then we come to Shankaracharya. Shankara himself did not really contribute anything new to the teaching of Vedanta not even a new emphasis as did Sri Krishna and Buddha; yet he figures very prominently in the history of Vedanta and is undoubtedly one of its greatest teacher-prophets. He was born in the south of India in the late eighth century. When he was just a boy he left home and wandered until he came to a teacher in central India of whom he had heard a great sannyasin and knower of God, whose name was Govindapada. The story is that he was waiting for this disciple, but probably that was an afterthought. In any case, when this boy came Govindapada was highly pleased and taught him everything that he knew, and under his instruction Shankara realized Brahman; that is to say, he attained to the highest knowledge. And then the teacher blessed him and before he passed away told him that he must reestablish Vedanta.
What did he mean by reestablish Vedanta?’ As I have said, there were the Upanishads a hundred and eight or twenty-eight or twelve or ten, as you may consider it which were the original texts of Vedanta philosophy and religion; then there was the Brahma Sutras or Vedanta Darsana and there was the Bhagavad-Gita, which contains the essence of the Upanishads. These three bodies of scripture are called the main sources of Vedanta, and Govindapada wanted Shankara to establish them on grounds that could not be assailed by anyone. You see, in those days there were great Buddhist and Jaina philosophers and strong leaders of other religions. The country had become, as it were, divided into many warring sects. In actual practice, there was not much religion left, but there was a lot of intellectual talk. The ancient knowledge of the Veda and Vedanta was lost, and it was left to Shankara to reestablish it. And that is what he did. He composed most extraordinary commentaries on ten main Upanishads, which are still unsurpassed by anyone. Even his opponents have not been able to find fault with any of his writings or with the way he has supported Vedantic views. There has not yet been a single error discovered in his arguments, which shows what a clear intellect he had.
Then he wrote a voluminous commentary on the Vedanta Darsana, and also a commentary on the Bhagavad-Gita, which is considered to be the oldest of all its commentaries and which is still the crown of the many commentaries written since. In addition, he wrote many independent texts, some of which are very well known, and composed many hymns and poems.
That is only his writing side.