More than half of England's schools are failing pupils on religious education, the schools watchdog Ofsted has said.
Its report accuses schools and the government of failing to focus effectively on the subject.
It adds that six in 10 schools are not "realising the subject's full potential" in an increasingly globalised and multicultural century.
The Religious Education Council for England and Wales called the findings disappointing but not surprising.
The report highlights low standards, weak teaching, weak examination provision and confusion about the purpose of RE.
In particular, it says the recent introduction of the English Baccalaureate measure for pupils who achieve grade C or above in English, mathematics, science, a language and either history or geography, ignores RE and has further marginalised the subject.
The report echoes comments by the Education Secretary Michael Gove. In July he told religious leaders RE had "suffered" because of government changes.
He said he had thought that because schools have a statutory duty to provide RE lessons the subject was protected.
Ofsted's report, Religious Education: realising the potential, says the subject "plays a key role in promoting social cohesion and the virtues of respect and empathy, which are important in our diverse society" but it finds "many pupils leave school with scant subject knowledge and understanding".
It adds: "Moreover RE teaching often fails to challenge and extend pupils' ability to explore fundamental questions about human life, religion and belief."
Inspectors visited a sample of 185 schools, both primary and secondary, between September 2009 and July 2012.
They found achievement and teaching in RE was less than good in six out of 10 primary schools, and in fewer than half of secondary schools and that not enough had been done to improve provision since a previous report in 2010.
Teaching in primary schools was "not good enough because of weaknesses in teachers understanding of the subject", they found.
Standards were higher for GCSE and sixth form students but even at GCSE level teaching often "failed to secure the core aim of the examination specifications, that is to enable pupils to adopt an enquiring, critical and reflective approach to the study of religion."
The authors urge the Department for Education (DfE) to consider whether the quality and quantity of statutory RE provision should be decided locally by schools and local authorities - or specified by central government.
They want the DfE to work with professional RE teaching associations to "clarify the aims and purposes of RE" and to improve teacher training.
Schools should ensure that teaching deepens "pupils' understanding of the nature, diversity and impact of religion and belief in the contemporary world," they urge.
John Keast, chairman of the Religious Education Council for England and Wales said they had been warning "for some time about the poor state of religious education in many schools".
He said they would publish their own report and RE curriculum later in October.
"It is now vital that the DfE works with the Religious Education Council on putting things right. We can do better than this."
Stephen Evans of the National Secular Society said the recommendations did not go far enough.
"With the freedom to determine their own syllabus for RE, many schools with a religious character abuse the subject and use it for missionising.
"Young people would be better served by a new national curriculum subject for all pupils that covers a variety of religious, non-religious and secular philosophies and world views."
Meanwhile, Gail Larkin from the National Association of Head Teachers said consecutive governments were responsible for schools placing less emphasis on the subject.
She said: "Some of the major policy changes that we've had, the impact has been to convey the message that RE is of less value than other subjects.
"So, as a head teacher... we are concentrating more - as we've been told to do by consecutive governments - on core subjects and subjects that can be measured."
"Unfortunately", she added, "I think RE has fallen off the radar".