I don’t think that non-belief automatically means that we can’t or shouldn’t participate in debate on the subject. Most atheists (and also believers) would probably intervene if they saw instances where religious or supersitious beliefs were significantly harming people’s lives: parents using faith healing on a sick child; fundamentalists putting down gays or saying that hurricanes are punishments from God; people who believe they are suffering from a voodoo curse or evil eye or have a haunted house, etc.
Debating with a person about their religion is like debating with a person about their choice of boyfriend or girlfriend. Most times there’s no reason to intervene or question people about their personal relationships or personal beliefs. On the other hand, if you see that a person’s relationship or religion is clearly toxic to them or others, I don’t think there’s anything in the “Atheist’s Handbook” that says we have to remain quiet about it.
In fact I'm a little surprised when people assume that I'm supposed to STFU about religion just because I'm an atheist. Why? Who made that rule, and why am I supposed to give a rat's ass about it? To me, being an atheist is about calling my own shots; it's not about following the rules in some imaginary "Atheist's Handbook."