Mohan Bhagwat of the parliament's right-wing Rashtriya Swayamsewak Sangh said he would ignore PM Narendra Modi's plea for the practice to stop.
India was a "Hindu nation" where many Hindus had been forcibly converted, he said.
"We will bring back those who have lost their way. They did not go on their own, they were lured into leaving."
The ruling Bharatiya Janata Party has condemned the practice of forced religious conversions and has said it will try and implement an anti-conversion law.
There are a small number of Christians in the predominantly Hindu nation.
Mohan Bhagwat added: "We don't want to convert anybody ... but then Hindus should also not be converted."