Christians "need to be wary" as the government weighs up changing legislation on terrorism and extremism.
The warning from The Christian Institute follows reports in The Telegraph that Whitehall is working on a new official definition of extremism amid a rise in anti-Jewish hate, and separate reports that Home Office officials are considering expanding the legal defintion of terrorism.
The Christian Institute's director Colin Hart said that the moves were understandable after the rise in antisemitism and scenes of celebration witnessed in parts of Britain after Hamas' terrorist attack on Israel. But he warned that any new extremism laws must not threaten religious freedom or remove vital protections to free speech.
He said "alarm bells should be sounding" about the reports and "get even louder" at the suggestion that an expansion of current definitions may include more groups and individuals who "create a climate conducive to terrorism, hate crime and other violence".
Such a redefinition "would be profoundly concerning in a context where a world-famous author can be branded a hatemonger simply for opposing self-ID sex changes".
"The recurring issue is overly broad drafting leading to unintended consequences. A legitimate problem may be identified, but the proposed solution is a sledgehammer to crack a nut," he said.
"We might all agree that specific behaviour needs to be stopped, but without precise wording innocent behaviour is caught too."
Mr Hart said that any changes to current definitions must protect the freedom to share the Gospel and not end up "censoring things that are merely controversial, unpopular or cause offence".
"Most importantly, they must not inhibit freedom to declare the Gospel, which carries its own unique offence," he said.
"That is the freedom we are currently privileged to enjoy and must be vigilant to defend."