I talk about religion (lower case 'r') with my (almost) 7yo often, but only because he asks. We (wife is a heathen also) explain what people believe, never in a derogatory or negative way, and that we simply don't believe the same thing. His grandma on wife's side is a JW, my mum is an elder in uniting church. He knows the basics of what they believe, and respects their beliefs (even the 'no Chrissy or birthday' thing on the JW side)
When he asked why they believe what they go, I tell him it's what makes them happy and helps them understand the world.
Btw, both my kids (other is 4), know, and are cool with our bodies turning back into fertilizer when he die. Wife's grandfather is on last legs at the moment, and I loved my 4yo reminding my wife that's it ok, because the worms will eat him.
My tips, never shy away from the questions, never ridicule or trivialize others beliefs (ridiculous as they are), and teach them to respect others beliefs (do as I say, not as I do). I try to explain, as best as I can, what science and nature has on offer to make sense of the world for them.
Whats HARD, is trying to explain to him why one if his friends loudly an frequently tell him that his mum says that ghosts are the tortured spirits of people who don't believe in god, and that's what's going to happen to him (my son) and his family (us). He doesn't believe it, but it baffles him that people would think and say such things...,