The Christian Institute is urging churches to get a handle on their legal rights after a local council recently suggested churches must perform same-sex marriages.
Essex County Council circulated a letter stating that churches "must be licensed" to conduct gay marriages "with immediate effect".
The council apologised a week later but the incident has prompted The Christian Institute to issue guidance reassuring churches that they are entitled to refuse.
'Same-sex marriage: Your legal rights to object' offers simple answers to 19 questions around gay marriage law and how it affects churches.
The Christian Institute's Simon Calvert said: "The behaviour of Essex County Council goes to show why churches need to know their legal rights, because bureaucrats who want to push for gay marriage will try and go beyond the law."
The booklet is free and provides advice for both Anglican and non-Anglican churches in light of the Marriage (Same Sex Couples) Act, which came into force in March.
It covers church provision of marriage services, church teaching and pastoral support, church policy and administration and Christians in the workplace.
"This free legal guide is a vital resource for the church in this land as it battles with a new law which goes against Biblical teaching," said Mr Calvert.
"We want to be clear that Christians still have the right to express their belief that marriage is between a man and woman. Christians have every reason to be confident and bold in upholding the truth about marriage.
"Looking long-term we have always had concerns about the effect of litigation in the European Court of Human Rights. There is instability in the law because there was never a safe way of introducing same-sex marriage.
"But there is no legal reason whatsoever for churches to stop holding marriages in the ways they always have. They are free to do so."
The guide is available to download here