Should ESL Classes Never Mention Religion?

As religion is an integrated part of human history and culture, an L2 cannot be learnt without some mentioning of it in the language classes. Teaching about religion cannot only help L2 learners learn the L2 itself but other general knowledge such as history, music and arts. However, differences should be made between teaching religion and teaching about religion and some guidelines should be followed if teachers are to teach it in the language classes.