You just don't show up, they arn't going to take "I just don't want to be a christian" as an answer. And for good reason...you may not see it their way but in their mind, they are trying to save you from what they believe to be hell.
So just stop going and put it behind you...if that's really your choice. But keep something in mind, lately there has been a lot of out reach for Atheism. Keep it to yourself. Don't go about putting down christians for what they believe. You may have lost your faith and hope, but don't ruin it for them, even if it isn't real. I know Christians get very annoying with all the outreach and the whole "Your going to hell"...but don't put yourself in the same situation. They believe that they are doing good by trying to "save" people. Athiests...I have no idea what good they think they are doing by trying to take away other people's faith.
*edit*
And I'd like to shout out to all you church goers out there who say "well then you weren't a real Christian". Because that's bull..and you people make me sad. Who are you to say who is a real Christian and who isn't? Who gave you the right to decide who's in God's favour and who isn't? He could have been a more Christian then any of you. You know why? Because something challenged him. If you havn't faced any serious doubt that God is up there, and if you havn't thought about the possibility that perhaps God ISN'T real. Then none of you are Christians. Because your faith is so weak and blind that it hasn't been tested. His has. And it just so happens that he chose to move away from it. So you can assume how strong his past faith was, but you will never know for sure. And telling him that he was never a "real" Christian is part of the attitude that makes atheists want to smack you across the face. For all the Christians who are pretending to be concerned for him, shut up. Don't tell him he's wrong and let him make his own choices. You know what you can do? Pray...but other then that..you're not helping at all. Atheism isn't what saddens me...it's the Christians that push people into that belief. As I said before, I hope that he doesn't become what I call an Evangelical atheist...but I wish Christians would realise that it's not their mouths that convert people. It's their actions if anything. So anyone else who would like to comment, please, keep that in mind.