Catholic schools cannot exist without instruction in the Catholic faith, a parents' conference heard on Saturday.
Mr David Hegarty, president of the National Congress of Secondary School Parent Associations (CSPA) said if the programme for religious education in schools developed by the National Council for Curriculum and Assessment replaced religious instruction, it would "violate parents' rights in canon law.
"Naturally our children should learn about the other religions, but only when they know the basics about their own faith," he said. "They will learn little or nothing about their own religion from this programme or the books designed for it.
"Without instruction in the Catholic faith for pupils, you do not have Catholic schools. Anything which undermines the status of this essential instruction is detrimental to the ethos."
Mr Hegarty said the CSPA had met with a "curious official silence" when it had sought "realistic" funding, adding: "We have not been granted a cent of official funding since we left the National Parents Council post primary."
Delegates to the day-long Dublin conference also heard a presentation from disability-rights campaigner Ms Kathy Sinnott on the delivery of special-needs services from a parent's perspective.
Ms Sinnott warned that the proposed new EU constitution could have serious implications for parents of children with special needs, as it moved away from the concept of natural law.
Whereas individuals had certain God-given rights under the Irish constitution that nobody could challenge, the new EU constitution sought to move away from this approach.
Other speakers included Mr Paul Coy, of the Crew Network, who made a presentation on a co-ordinated drugs-awareness programme for secondary schools.