A prominent theologian says parents are confused about the word "religion", which can make them more likely to pull their children out of religious classes.
But that means classrooms are now lagging behind international standards in terms of teaching children about the religions of the world.
Sir Lloyd Geering said a misunderstanding of religion in schools over the years had meant many people knew little about it.
"Great Britain is the only country in the world I know of where religion is [a] compulsory subject. It was felt that these were the sorts of things that were essential to pass on to children.
"Britain is far ahead of us in this respect. What is taught in British schools is what is taught here in universities as religious studies. But that's what should be done in the schools."
Sir Lloyd, emeritus professor of religious studies at Victoria University of Wellington, said the secular Education Act meant New Zealand's curriculum had been lacking, for some time, in a way that those in other countries had not.
The word "religion" had been putting people off.
"I think 'religion' is probably a word that is confusing the whole situation. What we really should be talking about is culture. Religion is simply the spiritual dimension of culture - every culture has a spiritual dimension.
"In most countries, there is some form of educating people in what are the important things of culture, which includes its spiritual dimension - commonly called religion.
"This is an aspect of culture that has been sadly lacking in our curriculum."
Sir Lloyd's comments come after it was revealed that a number of primary schools in Auckland have opted out of teaching the Bible in class because of a lack of interest and support from students and parents.
At least three schools in Auckland are understood to have stopped the classes. One principal told the Herald this week that the decision to stop classes came after 50 out of 500 students enrolled in the Churches Education Commission programme dropped out last year. More followed shortly afterwards.
David Hines, of the Secular Education Network, said some members of his group believed religion should be taken out of the curriculum altogether.
Others believed that a range of beliefs - such as Islam, Hinduism, Buddhism and even atheism - should be taught in schools, not just Christianity, provided they were taught in a fair and objective way.
Another issue they were being told about from various communities was the lack of religions being taught that reflected a particular group of students, such as Muslim pupils.
"All they want to be taught in schools is critical thinking. Then kids can work out for themselves which religion they want," Mr Hines said.
"All of us want schools to be fair and secular, but secular meaning there's no preferance to religion.
"Some would say religion should be taught about, in an objective way," Mr Hines said.
The Secular Education Network and religious experts plan to meet within the next few weeks to discuss the issue.