There are fears that the ruling Bharatiya Janata Party (BJP) want to change the law to make India a Hindu theocratic state.
Bishop Nazarene Soosai of Kottar Diocese, southern India says that would be a disaster and that Christians "fear the day".
He's been speaking to Catholic charity Aid to the Church in Need as the final round of voting is beginning in the nation's marathon election.
It has consisted of seven rounds over six weeks.
Prime minister Narendra Modi's Hindu nationalist party is seeking re-election for another five years.
Bishop Soosai said: "They say there is no other Hindu nation, so India must be the Hindu nation.
"They say we need to be a Hindu state in opposition to Pakistan which is Muslim and they think all the western countries are Christian - although they are really secular - and so they say we need a Hindu country."
Over 700 attacks against Christians were recorded in India in 2017.
The bishop says that will only get worse if the constitution is changed.
"Broadly speaking, religious minorities are under attack," he said. "Minority rights are under threat and much infringed.
"And when you raise your voice for human rights they say you're not a patriot."
The election results are expected later this week.
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