Best advice I can give is to put it in writing. Send him an email.
Produce an outline so that you can plan ahead what you want the email to say.
Open by telling him that you know that this has the potential to be a heated and emotionally charged discussion and, rather than risk it devolving into yelling and hurtful words, you'd like to keep the discussion in writing so that voices can't be raised and so that, hopefully, you can step away from the computer and take a good deep breath or two before hurtful things are said.
Begin the email by telling him that you're still the same person that you've always been. Cut off his source of fear when it comes to how you're going to live your life as an atheist. Point out that while you've been an atheist for a while, there isn't some point he can point to where you changed and stopped making moral decisions. Point out that you're still the good person he raised you to be. Point out that there isn't any reason to think, just going on the evidence of the past, that you're going to stop being the person you've been and that he raised you to be.
From there, tell him 3-5 of the major problems you have with the idea of god's existence. Try to imagine the objections he might raise to what you're saying and cover those objections right away so that there's less of an opportunity for a back and forth. Show him that you've given it a lot of thought. Explain how hard it was to come to this conclusion, how hard you fought to maintain your faith. And how you just couldn't do it in the end.
Hope that helps you!
To add on to Nelson's comment; The great thing about communicating through writing is it gives a) you a chance to write, re-write and clarify your feelings so when you send it, it is your clear and genuine thought and b) When people are listening and they hear something that is a "trigger" for them their logical thoughts process are put on hold while the emotions take over as their body goes into full defense/attack mode. At this point they are no longer listening, they are thinking of responding while you are still trying to make your point. whether in their head or out load by interrupting you. This makes communication difficult at best and futile at most. Writing allows it all to sink in. He may have to read it several times and each time, hopefully, he will begin to see it with a reasonable and rational heart.
Lastly be respectful of his desire to be a man of faith and make sure you know how much you love him.
I think you've had very good advice. In writing to your father you will not be able to be interrupted and also your natural anxiety at having to oppose your father cannot make you appear unsure of yourself. Good luck with it. :)
Firstly why do you need tell him?. Secondly if you choose to tell him (and its YOUR choice ) I would tell him face-to-face and thirdly you do not have to validate your beliefs. regards Bob
My father wasn't interested in why I was an Atheist, he just knew I was wrong.
Yeah, in my post above I should have been more clear about what I think the purpose of including some of the reasons for his atheism for his father would be. I certainly don't think that his father is necessarily going to be interested in those reasons, and I absolutely do not think the email is going to be an opportunity to convince his dad that theism is false, atheism true. I just view it as an opportunity to demonstrate to his father that he's given the question a great deal of thought and that he has some mature considered and thoughtful opinions on the matter.
My advice don't do it unless it becomes an issue. then do where its just the two of you. prepare a few talking point, he will try to reconvert you, so have a few good arguments prepared. if it gets to the point of an argument walk away and don't come back until he's ready to respect your opinions. also don't worry about hurting his feelings, hes a big boy he can handle, worry about him hurting yours. also the only way you be prepared for questions is if your well read on the subject, read the bible and take note of the immoral parts (book chapter and verse).
I agree with Nelson. Putting it in writing could avoid a heated discussion that goes off topic and could lead to personal attacks. When I told my mother she took it really personal and started to bring up tons of unrelated things trying to figure out what she did wrong. She still doesn't really understand and thinks I'm going through a phase. I think putting it in writing seams like a good suggestion for myself. It would be a good way for my mother to get my complete thoughts before interrupting me with her sobs and wondering out loud if she gave me too much freedom or not enough.
I've never really sat down and really talked to my mom about it. She learned through discussions I had with my Aunt (her sister) on FB. Not the best way to find out I'll admit. My mom has attempted to pull Pascals's Gambit on me.. "What's it going to hurt to believe?" .. My response was, "Well, assuming for a second God exists and he is all knowing. He'll know I'm not being genuine and essentially lying. Last I checked lying is one of the 10 Commandments.So I'd rather be honest with myself and with him -- again assuming they exist. If they don't, I haven't wasted my life in believing in something that's not real." That seemed to quiet her for that moment..Since then she's asked the question again on several different occasions, I assume thinking I'll change my stance. ... Just my own experience.
I don't know your father, so I can't predict how he will react. I think it's important to reflect upon the function of telling him that you don't believe as he does. After that, you can commit to your goal regardless of the difficulties that may arise. When I told my parents, I think my goal was mainly to stop living what felt like a lie and to stop doing things (like going to mass and praying) that I no longer wanted to do. While their reaction hurt me (and them) a lot at first, it was temporary and no worse than what I felt sitting in church with them. Parents with strong religious affiliations and strong beliefs are naturally going to be scared when their children venture outside of what they consider to be a safe zone. My only advice is to be clear about what this revelation means in terms of how your behavior in front of your father is going to change (e.g. no more sunday church, no more praying, if I marry it won't be in a church, etc).
Also: If he doesn't accept it right away, that's ok. I've had friends and family members who were shocked at first, but after a while, and after many questions and re-conversion attempts, finally have accepted things.
Remember, you've had time to come to grips with your atheism, but it may be shocking news to them and they may need time to process.