Swami Dayananda Saraswati was the first Hindu reformer to campaign for the abolishment of Sati or burning of Hindu widows mercilessly till death, and also campaigned for the eradication of casteism, untouchability, inequallity, suppression of women and other superstitions which had dehumanised the Hindus for centuries, and had no foundation in the vedic religion.
It is because of such evils that conversion of Hindus to Islam and Christianity became rampant in the preceding centuries and Dayananda helped to put an end to such practices.
Through the Arya Samaj, Swami Dayananda Saraswati humanised and rationalised Hinduism, and helped Hinduism to become a progressive religion rather than the regressive one it had been for many centuries.
S.Radhakrishnan, renowned scholar and former president of India expressed tribute to Swami Dayananda Saraswati and the Arya Samaj in this statement...
"Swami Dayananda ranked highest among the makers of modern India. He had worked tirelessly for the political, religious and cultural emancipation of the country. He was guided by reason, taking Hinduism back to the Vedic foundations. He had tried to reform society with a clean sweep, which was again needed today. Some of the reforms introduced in the Indian Constitution had been inspired by his teachings."
If Swami Dayananda Saraswati had appeared a millenium back and effected his reforms then, the foreign invasions of India by the mughals and colonial powers could have been prevented.
As a saying by Will Durrant goes, "A great civilization is not conquered from without until it has destroyed itself from within."