A row has broken out in India's parliament over a conversion ceremony held in Agra, Uttar Pradesh, for around 200 Muslims – and it highlights the increasingly contested nature of faith in the world's largest democracy.

The ceremony was organised by an offshoot of the Rashtriya Swayamsevak Sangh (RSS) nationalist organisation. However, according to campaigners, the 'conversions' were forced and the participants had no idea what they were doing. Others alleged that they were lured to the event by the promise of ration cards.

Opposition MPs attacked the BJP government, with the Congress Party leader in the upper house, Anand Sharma, saying: "The House and the country need reassurance that the Constitution will not be violated." A police investigation has begun.

The controversy follows another sparked by an attack on non-Hindus by a government minister for the ruling nationalist Bharatiya Janata Party (BJP). Sadhvi Niranjan Jyoti was forced to apologise for insulting non-Hindus by telling voters at a campaign rally in Delhi that they must "decide whether you want a government of those born of [Hindu god] Ram, or those born illegitimately". The incident plays to fears that the BJP's Hindu agenda will see minorities facing discrimination and persecution.

The ability to convert from one religion to another is a basic human right, according to the United Nations.

When someone decides to leave one religious community for another, it can be hard for family and friends to deal with. However, conversion can sometimes stand for more than an individual's personal choice: it's a battleground between beliefs, cultures, history, communities and nations, and the mixture can very easily turn toxic.

Nowhere is this clearer than in India, secular by its constitution but deeply religious in its national life. It is mostly but not overwhelmingly Hindu, with 80 per cent of the population following the religion; around 13 per cent are Muslim and just over two per cent Christian, and their rights are protected under the constitution.

However, the Hindu majority is increasingly active in attempts to convert – or 're-convert' – members of minority faiths to Hinduism.

The Agra event is by no means the first time Hindus have staged such conversion ceremonies – and it is not the first time foul play has been alleged. However, if plans by the RSS come to fruition, it will be dwarfed by another ceremony in the Muslim-majority city of Aligarh at Christmas. It plans to convert at least 1,000 Muslim and 4,000 Christian families from the Valmiki Dalit group, which it claims to have identified already.

The time and place were chosen deliberately. RSS spokesman Rajeshwar Singh said: "Aligarh was chosen because it's time we wrest the Hindu city from Muslims. It is a city of brave Rajputs and their temples on whose remains Muslim institutions have been established."

He told The Hindu that the Muslim ceremony would be on December 23, the anniversary of the martyrdom of a pioneer of the re-conversion movement, and that the Christians would be received on Christmas Day. Christmas was chosen as the day for conversion because the event is a "shakti pariksha" [test of strength] for both religions: "If their religion is better, they can stop them. It is a test for both of us. If they come to us on Christmas, it is the biggest rejection of the faith."

Singh said that within two or three years the rural hinterland of Uttar Pradesh would be "free of Christians".

However: like most "religious" conflicts, there is more to this than meets the eye. In India, decisions about which religion to follow are not purely theological. They are also influenced, particularly among the poorest, by financial and other practical considerations. The Valmiki are among the poorest and most marginalised in India. They work as "manual scavengers", collecting excrement by hand (outlawed since March 2014 but still practised), and in other low-status occupations. They face entrenched social and religious discrimination.

India's government has made huge progress in recent years in reducing caste differences, partly through positive discrimination in favour of 'Scheduled Castes" – groups historically disadvantaged and eligible for help in the form of affirmative action including guaranteed political representation. However, Christian "Dalits" – the name means 'broken' or 'crushed' and was adopted as campaigning self-description – do not have Scheduled Caste status and the government has just indicated that it does not intend to grant it to them. Given the grinding poverty and stigmatisation they face, even a very small improvement in their condition might seem worth having – though Roop Rekha Verma, a former Lucknow University Vice-Chancellor who runs an organisation promoting communal harmony, warns that this might be illusory: the Hindus who are converting them are still firm believers in the caste system which keeps them down, she says, and going back to Hinduism will mean they will "be used as fuel against minorities".

While so far, the current struggles over conversion have been confined to words, this is far from always the case. It was Hindu activists who burned to death missionary doctor Graham Staines with his two young sons in their camper van in 1999, accusing him of converting Hindus. In rural areas especially, Christians are vulnerable.

Hinduism has a reputation for being a tolerant faith, and this is very largely deserved. However, faith in India is a contested space, with many Hindu nationalists seeing Christianity as an alien import in spite of the length of time it has been established there. So every conversion to Christianity is a blow to national pride, and every conversion to Hinduism is a victory.

Christians are very familiar with this instinct for religious empire-building, and too many evangelistic organisations are still in thrall to it today. However, for Christians, Muslims and Hindus alike, moving beyond worldly motives to a genuine respect for the spiritual journey of the individual – even if it takes them in the 'wrong' direction – is vital if there is to be peace between communities.