The victory of the Bharatiya Janata Party in India last month was followed by expressions of concern for Christians and other minorities because of the party's links to violent Hindu nationalism.
Those fears have been further compounded by fresh reports that the BJP's vice chairman told senior politicians in a recent unofficial visit to Nepal that religious conversions should be banned.
According to Christian Solidarity Worldwide (CSW), BJP vice chairman Bhagat Singh Koshiyari called for an immediate and complete legal ban on conversions during his visit to the country from 30 May to 1 June.
He reportedly alleged that Western countries are assisting Nepal in converting Hindus to Christianity.
Koshiyari made the comments during meetings with Prime Minister of Nepal, Sushil Koirala and Chairman of the Maoist party, Pushpa Kamal Dahal.
The timing of the call is sensitive as Nepal's Constituent Assembly is in the process of drafting a new constitution. It follows Nepal's 2006 shift from being the world's only official Hindu state to becoming a secular republic.
CSW says the parliament is under pressure from some political quarters to restore the Hindu monarchy.
But the human rights organisation is urging the country to make sure the constitution upholds human rights, including religious freedom.
"Mr Koshiyari's comments are of concern given that they appear to be an attempt to persuade Nepali leaders and political parties not to include freedom of religion or belief in the new constitution and laws of Nepal," said CSW's chief executive Mervyn Thomas.
He appealed to Nepal to remember its treat obligations, including Article 18 of the International Covenant on Civil and Political Rights, which protects the right to change one's faith.
He said the protection of this right was "an essential part of any constitution which adheres to international human rights principles".
"CSW continues to urge all political and religious leaders in Nepal to ensure that the new Constitution and Penal Code guarantee all citizens the right to express and share their beliefs, the right to choose and change their religion or belief, as well as the right not to believe in any religion," he concluded.