Question
Why do Hindus bathe in river Ganges (Ganga)?
Answer
King Bhagirath did Tapas to bring the Holy Ganga to earth in order to wash the ashes of his forefathers and free them from sin. Bhagirath's wish was granted and Ganga was released from the Heavens to earth. But the great speed with which Ganga descended to earth was capable of destroying everything on its way. So, Lord Shiva, to save the world, caught Ganga on his long hair and released it with reduced speed.
Due to this, the already holy Ganga was considered sacred since she descended directly from Lord Shiva's head. In olden times, people normally used to bath only in ponds, lakes and rivers (may be bathrooms were not prevalent during those days). Here, we must note that the very reason of bringing Ganga to the earth was to wash off the sins of the forefathers of Bhagirath. So, the people who normally bathed in lakes and rivers showed a great interest in bathing in the Holy river Ganga, with the idea that it will wash away their sins too.
However, taking bath in Ganga alone is not capable nullifying one's sin. This can be understood from the following argument between Lord Shiva and Goddess Parvati:
Once Lord Shiva and Goddess Parvati were traveling on earth. Goddess Parvati saw thousands of people heading towards River Ganga to take holy bath as it was an auspicious day.
Goddess Parvati wondered whether all these people will attain Moksha or liberation by bathing in River Ganga. So she asked Lord Shiva – do you think all these people heading towards Ganga will attain Moksha?
Lord Shiva laughed and said that only a dozen or so among them will get Moksha. Only that person who has come with devotion and pious mind will get Moksha. Only those persons who will not commit any sin after the holy bath will get Moksha. Most of these people that are waiting to take a holy dip have come to perform it as a ritual and get over with a responsibility. Some are here to show off that they are religious minded. Many of them are here to satisfy their ego of bathing in Ganga. Shiva said that one who is really yearning for Moksha will attain it here on this earth itself and for such people taking a dip and not taking one is of same value.
But Goddess Parvati argued that all the people here have come from far of places and they are spiritual, good and religious minded.
Shiva agreed to prove that He was right. Shiva disguised as a leper and Goddess Parvati disguised as his beautiful wife and went to the banks of Ganga. Goddess Parvati as wife started asking people to help her husband perform the Ganga Snan so that he will be cured of leprosy.
Majority of the people ignored her and went to take the holy dip. Some said some sentimental words and showed sympathy but did not help the leper perform the holy bath. Some advised the beautiful wife to leave the leper there and go home and get re-married. Some passed vulgar comments. Some asked her to get married to them.
After several hours and thousands of people passing by, a man came and helped the leper perform the holy bath. He remained silent and just said namaste to the wife and leper and went away.
Lord Shiva then said to Goddess Parvati that the man who helped the leper has already attained Moksha. He has already realized that all that is here is nothing but the Supreme Being. He is silent, calm and serene. For him a leper and handsome man is the same.
Argument between Lord Shiva and Goddess Parvati cited from http://www.hindu-blog.com
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