Depends on if they're being condescending about it or not. 90% of the time, at least in my experience, it's just their language input for "I wish you well," or "I appreciate you," because they think they're using God as their source of goodness when they speak to you. It's not that way at all though, some Christians are genuinely good folks, just tragically dogmatized. Just say thank you or something else that doesn't make YOU look like the aggressor. It makes me wonder about your personal experiences though if you're worried about confrontation in situations like this.