Three families were seeking judicial review of the government's decision to give priority to religious views in the new course, due to be taught from 2016.
But government lawyers argued equal consideration for religious and non-religious views is not required by law.
Mr Justice Warby said he would give his judgement at a later date.
Changes to the content of religious studies GCSE, announced in February, sparked complaints at the sidelining of non-religious world views, with former Archbishop of Canterbury Rowan Williams among 28 religious leaders who urged the government to rethink its decision.
The three families argue that Education Secretary Nicky Morgan has taken an unlawful approach to the subject and is failing to reflect the pluralistic nature of the UK.
Their QC, David Wolfe, told the High Court there was widespread concern "about the secretary of state's failure to comply with her duty of neutrality and impartiality as between religious and other beliefs".
But lawyers for the education secretary argued neither statutory provisions nor the European Convention on Human Rights requires equal consideration to be given to religious and non-religious views in the curriculum.
They said that although some schools rely on the religious studies GCSE to discharge their duty to provide religious education for 14- to 16-year-olds, provision has been made for non-religious beliefs to be studied and a school's curriculum is a matter "for local determination" by individual school authorities.
The three families come from Cumbria, Kent and Somerset.
The case is backed by the British Humanist Association.
The government has "skewed" religious studies teaching in England by excluding non-religious beliefs from the new GCSE, the High Court has heard.