Four British Christians who claim they lost their jobs as a result of discrimination against their beliefs are taking their cases to the European Court of Human Rights.
They include an airline worker stopped from wearing a cross and a registrar who did not want to marry gay couples.
All four lost separate employment tribunals relating to their beliefs.
Secular critics have said a ruling in favour of the group could "seriously undermine" UK equality law.
A ruling is not expected from the European court for several weeks.
The cases involve:
- Nadia Eweida, a Pentecostal Christian from Twickenham, south-west London, who was sent home by her employer British Airways in 2006 after refusing to remove a necklace with a cross
- Devon-based nurse Shirley Chaplin, who was moved to a desk job by Royal Devon and Exeter NHS Trust Hospital for similar reasons
- Gary McFarlane, a Bristol relationship counsellor, who was sacked by Relate after saying on a training course he might have had a conscientious objection to giving sex therapy advice to gay couples
- Registrar Lilian Ladele, who was disciplined after she refused to conduct same-sex civil partnership ceremonies in north London
Each individual had made a separate application to the court, but the cases are being heard together.
Miss Eweida's lawyer, James Dingemans, said her employer had permitted other religious symbols to be worn.
He said: "She was working alongside colleagues who were able to wear religious symbols and attire including the Sikh turban, the Sikh bracelet, the Muslim hijab, and the Jewish skull cap.
"It was indisputable that wearing the cross visibly did not have any detrimental effect on Miss Eweida's ability to do her job."
But a lawyer for the government, James Eadie, said employees' rights have to be limited in order to protect the rights of others.
He said: "These four linked cases at their core raise questions about the rights, and the limits to the rights, of employees to force their employers to alter employment conditions, so as to accommodate the employees' religious practices.
"My submission will be that the court's jurisprudence is clear and consistent, it is to this effect the convention protects individuals' rights to manifest their religion outside their professional sphere.
"However, that does not mean that in the context of his or her employment an individual can insist on being able to manifest their beliefs in any way they choose. Other rights, other interests are in play and are to be respected."
'Right to religion'
Court documents explained that Miss Eweida and Mrs Chaplin believed the UK law has "failed adequately to protect their right to manifest their religion" which is contrary to Article 9 of the European Convention on Human Rights.
This article provides a right to freedom of religion, including to worship, teaching, practice and observe elements of their faith.
They also claim that previous tribunal rulings have breached Article 14 of the convention, which outlaws discrimination based on religion.
Miss Ladele also believed her right to an "effective remedy" was infringed, and Mr McFarlane claimed his right to a fair trial and right to a private life in the UK were breached.
Earlier this year, the UK's equality watchdog, the Equality and Human Rights Commission, said the UK tribunals had come to the correct conclusion in the cases of Miss Ladele and Mr McFarlane.
But it conceded that the courts "may not have given sufficient weight" to Article 9.
Andrew Marsh, campaign director at religious group Christian Concern, whose sister organisation Christian Legal Centre is supporting Mrs Chaplin and Mr McFarlane, told the BBC the four could have had their beliefs respected by their employers without adversely affecting the people they serve.
"The crucial question in these cases is this: could these four individuals have been reasonably accommodated and their Christian faith respected, without detriment or damage to the rights of others - and the answer to that question is clearly yes.
"Each of them could have been reasonably accommodated without there ever being any danger of risk, significant risk to others or indeed of anyone who is entitled to a service being denied that service."
However, the National Secular Society - which campaigns against "religious privilege" - said a European court ruling in favour of the quartet would undermine UK equality law.
Society director, Keith Porteous Wood, said the group was fighting the action: "We think that if it goes the wrong way it will cause a hierarchy of right, with religion at the top, and it's going to be bad news for employers and for gay people."