Leaders of 100 black churches are due to stage a rally at Westminster on Monday against the government's Religious Hatred Bill.
Muslim groups have been prominent in seeking a religious hatred law
Church spokeswoman Katei Kirby said they believed the bill could hinder their freedom to preach.
"What the bill offers is a chance to gag people who want to proclaim their faith confidently," Ms Kirby said.
The Racial and Religious Hated Bill would create a new offence of incitement to religious hatred.
It would apply to comments made in public or in the media, as well as through written material.
But the government has said it would not prevent proselytising.
Rally organisers say up to 5,000 black church members will lobby MPs as the bill has its key third reading.
Ms Kirby, general manager of the African Caribbean Evangelical Alliance (ACEA), said they had no objections in principle to anything which protected people's right to choose their faith and express their faith.
But the proposed legislation might affect churches' ability to "share the gospel", she said.
'No idle threat'
"At the moment, for example, we might do door-to-door stuff, leafleting or run seminars or events and invite people who are not Christians already to hear the Christian faith explained.
"Under this new law we risk seven years imprisonment if we do that if somebody takes offence."
In June Home Office Minister Paul Goggins said he did not expect many prosecutions under the new laws but it was important for parliament to send out a clear message.
Jews and Sikhs are already covered by race hate laws and Muslims groups have been pushing for similar protection.
The only group which wants this restriction is a tiny minority of Muslims who have a vested interest in censorship and a fear of free speech
Rally organiser Reverend Alfred Williams of the Christian Faith Tabernacle Church in east London warned that Labour MPs risked alienating a traditionally Labour-voting community if they supported the bill.
"Labour MPs should not doubt us. This is no idle threat. We will never support the party again if it passes a pernicious law wanted only by extremists who would rob us of free speech.
"The only group which wants this restriction is a tiny minority of Muslims who have a vested interest in censorship and a fear of free speech," Reverend Williams said.
Research suggests there are more than 4,000 black-majority churches in the UK and a quarter of black people regularly attend church compared to 7% of the population as a whole.