We all know Adi Sankara as the Advaita Sthapaka - the one who established Advaita, the concept of non-duality, which propounds that we can recognise our oneness with the Universal Spirit by adopting the Gyana Marga or path of inquiry and knowledge. The broad-minded Adi Sankara also realised that common people cannot associate themselves as easily with such a deep philosophy as with wordly practices and emotions. Therefore, he also recommended Bhakti as a means of conditioning one's mind and focusing on one's personal God.
This was at a time when Hinduism as a way of life was floundering and losing its erstwhile undipusted following, due to the rapid advance of non-Vedic religions like Buddhism and Jainism in India. But what Adi Sankara found more objectionable were the inappropriate forms of worship adopted by people who "claimed" to be Hindus. Several communities in India like the Kapalikas, Vaishnavas and the Shakti cult, were forgetting the core Vedic principles and focusing more on irrelevant and irreverent forms of worship such as bali (sacrifice of living beings), branding of religious symbols on the bodies of devotees, and so on. What was shocking was that these were all being done in the name of Hinduism, which is in fact a way of life founded on the principle of non-violence.
Therefore Adi Sankara realised that it was time to identify the core cults in Hinduism and align their ways of worship with the Vedic principles, so that even those who were not enlightened enough to realise the principles of Advaita would at least be expressing their Bhakti in peaceful and commendable ways rather than adopt violent methods.
Sankara, at this time, identified that the major cults could be identified in line with six principal deities of the Hindu pantheon - Ganapathyam (the worship of Lord Ganesha), Kaumaram (the worship of Lord Kumara or Karthikeya), Sauram (the worship of the Sun God - nature worship), Shaktham (the worship of Goddess Shakthi), Shaivam (the worship of Lord Shiva), and Vaishnavam (the worship of Lord Vishnu). These six major streams of Hindu worship are referred to as Shanmatham. Adi Sankara found that these had lost their sheen because they had moved away from the Vedic path and adopted crude ways of worship. He therefore took it upon himself to re-establish these faiths to their former glory.
He achieved this by travelling widely all over the country, debating with the heads of each of these cults, accepting their beliefs while at the same time aligning their thoughts with the ancient Vedic treatises and streamlining the expression of their Bhakti in the form of elegant modes of worship. Not only were the heads of these cults convinced by Sankara's arguments, they also ended up as his disciples. In the words of Goldsmith, "Those who came to scoff remained to pray." These disciples of Adi Sankara also traversed the nooks and corner of our country to re-establish the peaceful ways of worship advocated by Hinduism. It is believed that Adi Sankara decided to liberate his spirit from his body only after all these devotees returned and reported to him that Shanmatha had be re-established to its former glory.
In honour of the mammoth task achieved by him, Sankara is known as Shanmatha Prathisthapaka.
My sister's dance teacher lent me a book called "Sankara and Shanmatha", a souvenir published during a conference on the subject in 1969. One line in the book, about a particular teaching of Adi Sankara's was very inspiring: While Shivam (peace) is the Supreme Brahman whose realisation is open to all, Lord Shiva, is only one of It's many manifestations. Nothing can explain Advaita better.