There is also a corresponding group for the unbelieving "children" (regardless of actual age, obviously) here: http://www.facebook.com/groups/406543069389528/
I'm not sure exactly how to start this post, since it's a response to the troubles that some of my friends (in various locations) have run into recently, and I really don't want to name names or even talk about specific situations. But it seems worthwhile to look at this topic again, only this time specifically in terms of family relationships.
Before I jump in, I'd like to point out some other resources that might also be helpful:
- Friendly Evangelism - a while back, I wrote a series of posts on how Christians can talk to ex-Christians and non-Christians without driving them away, starting arguments, or giving offense. (It's best to scroll down and start at the beginning.)
- Alise... Write! - Alise is a Christian whose (formerly Christian) husband lost his faith. She has some very interesting material on moving into and being part of an "unevenly-yoked" marriage. (Again, it's best to scroll down and start at the beginning.)
- Better a good atheist than a bad Christian - John Shore talks about priorities.
- What to do if your college-aged child turns his back on Judaism - Shula J Asher Silberstein's article is aimed at Jewish parents, but there's plenty there to help Christian parents as well.
No. It's not fun. In fact, I can pretty well guarantee that it's not fun for anybody. The best you can hope for - the absolute best - is that it won't be a very big deal. That's true whether you're the Christian parent, or the atheist/agnostic child. It's a difficult, tricky situation. Which brings me to my first piece of advice:
Don't panic. I know this is a huge shock for you. I realize that - depending on your particular flavor of Christianity - you may be terrified by the thought that your own flesh and blood is now bound for Hell. You may be wondering how this happened, what went wrong, whether you could have done something to prevent it, and what happens next. You may feel that the world is out of balance, that everything is wrong. But whatever you do, share as little of this reaction as possible with your child. Wait. Walk away and have your primal scream in private.
Remember that however difficult, unpleasant, and horrifying this may be for you to accept, it was at least that hard for your child, too. In fact, it was probably harder. That feeling that someone just pulled the rug out from under you, or kicked a leg out from under your chair, or punched you in the gut? They've had that. And odds are, if you're reading this, they've felt that in very recent memory.
After that, watch your language. Losing your faith is almost never a quick, casual decision. For most people, it's an uncomfortable and unwelcome conclusion to a long and painful search for answers. So anything you say, or ask, that sounds like your child chose to quit being Christian is going to be unwelcome at best. At worst, it's infuriating.
Similarly, anything that has to do with their relationship with God is probably unwelcome. People who are unhappy with their church (or their minister, or their faith community) just go find another church. People who are dissatisfied with Christianity itself but still believe in some sort of divinity will move to another religion, or become "spiritual but not religious". To get all the way to atheism, or even firm agnosticism, you have to conclude that either God doesn't exist, or at the very least that He isn't active in the world. And once someone has reached that point, they don't have feelings about God. They don't see religion as having anything to do with God: it's just people. As far as they can tell, it has always been just people. So they don't "hate God" and they aren't "angry at God." They can't be; for atheists, that's like being mad at Santa Claus.
That said, they can be, and probably are, angry about being told for years about God. Once someone has concluded that God doesn't exist - or even that He isn't what they were told - they tend to feel like a great many people have lied to them. They tend to feel like they've been used and manipulated. They feel betrayed. If you're seeing anger, that's where a lot of it is coming from. That's not all of it, but that's a lot.
The other part of the anger is mostly - one way or another - a reaction to pain. Losing faith is painful. Losing faith means asking questions that peers, family, and authority figures may find uncomfortable, unwelcome, or inappropriate. (It feels a bit like admitting to people that you have a communicable disease.) In the process of losing their faith, a lot of people also lose friends, relationships, even entire communities. At its worst, losing your faith is like losing your whole world - or, worse, losing yourself. It's an experience of finding out that you aren't who you thought you were, that things you'd always relied on weren't true, or weren't there. This experience is made worse by the fact that very few people understand (and fewer accept) what the disaffected Christian is going through as they lose their faith - especially in more conservative, religious communities.
Basically, you should try to avoid anything that trivializes their loss of faith: anything that makes it sound like they're just being silly or childish or petulant; anything that makes it sound like a simple choice; and - and here's the hard part - anything that treats their lack of belief as something other than a lack of belief. Telling a former believer that they just had a bad experience with the church, or that they're just mad at God, sends a very clear message that you have no idea what happened to them and aren't interested in finding out.
Listen to people who know. As a rule, Christianity doesn't really prepare its adherents for the idea of people losing their faith. Changing from one denomination to another, sure - but dropping out of Christianity entirely? Rare few churches ever talk about that. Most churches assume that atheists have never heard of the Gospel, or at the very least that they've never been exposed to true Christianity. A casual reading of the Apostle Paul would suggest that there's no such thing as an atheist, that God's presence is so unambiguously obvious that anyone who denies His existence must clearly be in rebellion against Him. There are several problems with these views, but even if they were irrefutably true, here and now they're just not helpful.
By the same token, talking with your minister (priest, pastor, whatever) might be helpful, but odds are good that they don't have a lot more experience with this than you do. Despite what you (and they) might hope, their training isn't likely to be especially helpful, either. So by all means ask them for advice, but take their answers with a grain of salt (or, in some cases, an entire salt mine).
You know who's going to best understand what your child is going through? That's right... it's your child. Listen to what your child has to say about his or her experience. But - and this is a very big "but" - remember that losing faith is a process. It's not like buying a car, where one day you can just decide to go out and do it, and then it's done. Your child may not be able to fully articulate everything that went into their loss of faith, particularly not in a way that make sense to you. So listen. Ask questions if you must. Take time to think over what they tell you. Above all, do not demand answers. If you can accept that you may never understand what happened, that will probably help.
Treat your child like an adult. The age of the child is going to make a big difference, here. High-school or college age children may just be "going through a phase" or "rebelling" or what have you. Then again, they may not. I myself wandered away from Christianity in my early teens; I'm creeping up on forty now, and Christian beliefs still don't make any sense to me. Either way, it's best to treat your child as if this is a serious conclusion that they've legitimately worked their way to.
That's doubly important if your children actually are independent adults. I shouldn't even have to say that, but I keep running into parents who can't seem to process the fact that their children can, in fact, make decisions and reach conclusions on their own. Despite the fact that these "children" are completely self reliant, gainfully employed, married, and/or parents in their own right, their parents either can't or won't acknowledge that they have the right and ability to be self-determining.
Be ready to make some adjustments. If your child is still at home, forcing them to go to church with you isn't going to magically turn them back into a Christian - in fact, it's rather more likely to drive them further away. The same goes for leaving tracts around for them to find. If you, or they, aren't comfortable talking about religion - or can't discuss it calmly, or whatever - then put the topic off-limits. (Sort like the "Don't talk politics at the family gathering" rule. Remember that one? The one that was put in place because Uncle Charlie loves to argue, and holds political opinions that are diametrically opposed to those of everyone else in the family, and can't or won't pull back before things get really unpleasant?) If your grown child isn't comfortable with taking your grandchildren to church, let it go. It's not that important, and it's not like they're going to grow up never hearing about Jesus. Take a deep breath, be flexible, and try to focus on what's really important: your relationship with your child.
Above all, have faith. I know that sounds funny coming from me, but I'm serious. If you can't trust in your child, trust in God. Do you really think He's just going to let them fall? (If you just said yes, are we talking about the same deity? He'd sacrifice His only Son to save everyone, but you think He's just going to stand by when it comes to your child?) Do you really think He's all that concerned about whether or not they're aware of His presence? Especially when compared to, say, how they're living their lives?
Don't let your confusion and fear try to tell you the limits of God's grace and mercy. Have faith. Trust.
So that's my advice. I hope it helps. Comments are open. If there's something you'd like to add - something I missed, or something you think I got wrong - please contribute. If you have questions, please ask. If you've found other resources helpful, let us know. Discovering that your child has lost his or her faith is a difficult, unpleasant situation, but you can work through it and you can keep your family intact.
 At worst... well, I know of one case where a high school senior was kicked out of his house, his possessions tossed out on the yard to get rained on; not just disowned, but disavowed entirely.
 A quick note on terminology: "atheism" is generally defined as the belief that there is (or are) no God (or gods). Agnosticism is generally defined as uncertainty about the existence of God, or sometimes as the belief that it's impossible to really know whether or not God exists. In practice, there's a huge amount of overlap between those two positions. There are a lot of atheists who prefer to define the term not as definite belief that God does not exist, but as a lack of belief that God does exist. There are plenty of agnostics who are functionally atheist: since they see no definite evidence proof of God's existence, they assume that He doesn't exist.
 I realize that if you've looked this up on the Internet, it's probably just exactly too late for this advice. I stand by it nevertheless.
 Oddly, a lot of people find this easier to accept than the notion that their child doesn't believe in the supernatural at all. I say "oddly" because from a theological perspective, it makes no sense: in most Christian doctrine, a Buddhist is just as damned as an atheist. Though in all honesty, I think this particular part of Christian doctrine is based on a seriously misguided reading of the Bible.
 So my parents have been coping with my lack of belief for quite a long time, now.
 Seriously, in modern, Western nations, that's essentially impossible. Everybody hears about Jesus.