Lessons can lack "intellectual development", said Dr Nigel Fancourt of Oxford University.
His comments follow a poll finding widespread support in England for teaching about Christianity in schools.
Richy Thompson of the British Humanist Association said schools should also reflect non-religious world views.
Almost two-thirds (64%) of adults questioned for Oxford University agreed pupils must know about Christianity to understand English history.
Some 57% of 1,832 adults polled agreed that learning about Christianity was essential for children to understand English culture and way of life and 44% said more attention should be given to its teaching.
More than half (58%) said it was important for children to know about the history of Christianity, major Christian festivals (56%) and how it distinguishes right from wrong (51%).
The poll is part of a project to help schools teach the faith in a more rigorous way. Researchers from Oxford's department of education say their focus on Christianity stems from a legal requirement that English schools should reflect the fact that Christianity is the country's main religious tradition.
This means that Christianity "will probably be the only religion that pupils study throughout their schooling. It is treated in the same way as other religions but studied more frequently", said Dr Fancourt.