The European Court of Human Rights is due to deliver a landmark ruling in the cases of four British Christians who claim they suffered religious discrimination at work.
They include BA worker stopped from wearing a cross and a registrar who refused to perform same-sex ceremonies.
The four insist their right to express their religious beliefs was infringed.
The government, which is contesting the claims, argues their rights are protected only in private.
Campaigners on both sides say the judgement could shape UK equality law.
The cases involve nurse Shirley Chaplin, 57, and British Airways employee Nadia Eweida, 60, who took their cases to the European Court of Human Rights after being made to stop wearing necklaces with a cross by their employers.
Judges will also rule on the cases of Gary McFarlane, 51 - a marriage counsellor fired after saying he might object to giving sex therapy advice to gay couples - and registrar Lillian Ladele, who was disciplined after she refused to conduct same-sex civil partnership ceremonies.
All four lost separate employment tribunals relating to their beliefs and made individual applications to the court, but their cases are being heard together.
They say their employers' actions went against articles nine and 14 of the European Convention on Human Rights, which protect their right to "freedom of thought, conscience and religion" and prohibit religious discrimination.
Lawyers for the government argue employees' rights have to be limited in order to protect the rights of others.
Director of the Christian Legal Centre, Andrea Minichiello Williams, said: "These are landmark cases and we have waited a long time to get to this point.
"At stake is not only the future shape of Christian involvement in community life, but the protection of important personal freedoms in a diverse society."
Keith Porteous Wood, executive director of the National Secular Society, said the future of equality legislation in Britain - and even Europe - could depend on the ruling.
"We believe any further accommodation of religious conscience in UK equality law would create a damaging hierarchy of rights, with religion at the top," he said.
Former Archbishop of Canterbury, Lord Carey, has publicly backed the four Christians.
Ms Eweida, a Coptic Christian from Twickenham in south-west London, was asked to leave her job in 2006 after refusing to remove her cross.
An employment tribunal found she had not been subjected to religious discrimination, but BA later altered its uniform code to allow symbols of faith, including crosses.
Ms Chaplin, from Exeter, was transferred to a desk job by Royal Devon and Exeter NHS Trust Hospital for failing to remove a confirmation crucifix on a small chain, which she had worn to work for 30 years.
Ms Ladele was disciplined by Islington Council, in north London, after saying she did not want to conduct same-sex civil partnership ceremonies. Her lawyers said the service could have been performed by other employees who were prepared to carry them out.
Mr McFarlane, a Bristol relationship counsellor, worked for the Avon branch of national charity Relate but was sacked for gross misconduct in 2008 after saying on a training course he might have an objection to discussing sexual problems with gay couples.
A ruling from the European court is expected at 09:30 GMT.