The call has been made by The Union of Welsh Independents’ Council (UWIC), which represents more than 400 churches across the country.
Union leaders said at a time “when religious issues dominate the news headlines”, it is disappointing that not every pupil has the chance to study the subject.
In Wales, as in England, religious education is not part of the national curriculum, but is part of the basic curriculum for all registered pupils and is a statutory entitlement for pupils who are aged five to 19.
The religious education and collective worship that takes place in maintained schools in Wales is decided by each local education authority (through its SACRE – Standing Advisory Council for Religious Education).
Addressing a meeting of the council at Gregynog in Powys, the Reverend Aled Jones, the UWIC training coordinator in south Wales, said: “The Union of Welsh Independents’ Council appreciates the work done by religious education and religious studies teachers throughout Wales and congratulates pupils on receiving excellent results at GCSE, AS and A level exams annually.
“But we’re disappointed that not all secondary schools choose to offer religious studies as a subject at GCSE and A level. This is depriving thousands of young people in Wales from having the opportunity to consider some of life’s great issues.
“Religious studies discusses these issues by looking at the religious faith and conviction of people worldwide.
“Therefore, we’re asking every secondary school in Wales to ensure that their pupils have the opportunity to study for the full range of examinations in religious studies.
“We also call on SACRE members on the local authorities to influence schools to achieve this.”
In 2013, Estyn found that attainment in the full GCSE course in religious studies had risen steadily over the previous five years, with the percentage of pupils attaining grades A* to C well above the average for other subjects.
In response, a Welsh government spokesman said: “It is a statutory curriculum requirement that all pupils in Wales should study religious education until the age of 16 and Professor Graham Donaldson’s recently published report on a curriculum for Wales recommends that this should remain the case.
“However the decision on whether to study for a qualification in religious education is one for schools and individual learners and is not something that should be dictated by Welsh government.”