Shuddhi Movement was started by Arya Samaj in earlier part of 20th century to bring back the people who transformed their religion to Islam and Christianity from Hinduism. The literal meaning of Shuddhi
is purification but Arya Samajis didn’t aim at literal meaning rather they meant reconversion by the term.
There was vast difference in the scriptural authorities of these religions as Islam and Christianity claimed divine authority for their Koran and Bible respectively while Vedas of Hinduism didn’t put up such claims. Founder of Arya Samaj, Dayananda then strived to give the Vedas the similar degree of supernatural authority.
This process of reconverting Hindus from Islam and Christianity was accompanied by a Sanghathan movement.