(Before I go on, I think it's important to discern whether the person asking the question is really looking for an answer, or merely a debate. I won't debate my religious beliefs. I don't actually think you can argue most people closer to God. )
Having said that, it's important to think about what you really believe, and why, so that when someone who is sincerely searching asks you a question, you are able to provide a thoughtful answer. "I don't know," or "Just because," aren't particularly helpful, or likely to promote any further discussion or growth.
I was recently sent a copy of The Questions Christians Hope No One Will Ask, by Mark Mittelberg to review, and I devoured it. Truly, these are the questions that catch you off guard, questions about evolution, whether Jesus was really the Son of God, or merely a good teacher, about the hypocrisy among our own ranks.
I found Biblical answers for a few of my own questions, as well as frankly better, more in depth explanations of things I already believed. The explanations were accessible, obviously based on scripture, but also backed up with science, where applicable.
I think this would make a thought provoking Sunday School or small group study.
One of my close friends has recently become a Christian and has been asking me some hard questions. I'm torn between loaning her my book, and buying her a copy, so I can keep mine. :)
I loved this one.
Click here for a PDF of the first chapter.
Disclaimer: I received a review copy of this book from Tyndale. No other compensation was provided, and the opinions stated here are my own.