Senior religious leaders in south Sudan have called on people to vote for independence in a referendum to be held January next year.
The vote was promised as part of a 2005 deal to end years of war between the mainly Muslim north and the south, where Christianity is common.
"The way to unity is destructive," Bishop Paul Yugusuk said.
He said southerners would be treated as second-class citizens if Africa's largest nation remained united.
The BBC's Peter Martell in Juba, the capital of the south, says support for independence on the streets of the city is high.
However, the south's former rebel-turned-ruling party - the Sudan People's Liberation Movement (SPLM), is bound by the terms of the 2005 deal not to campaign openly for secession and ensure people make the choice for themselves.
But Bishop Yugusuk, of the Episcopal Church of Sudan, said this was something church leaders are not willing to do.
"We should not leave them to decide for themselves," he said.
"If they decide for unity... it will be disastrous."
Our reporter says the religious coalition, called the Sudanese Religious Leaders Referendum Initiative, brings together senior bishops from eight major churches, including the Roman Catholic and several Protestant denominations.
It also includes a Muslim leader from Central Equatoria State's Islamic Council.
An estimated 1.5m people died in the civil war between north and south, which is separate to the conflict in Darfur to the west of Sudan.