Keeping your religion (or lack thereof) to yourself

A good acquaintance of mine recently "converted" to atheism.  Having grown up in the church, he apparently decided that it wasn't for him.  I have to admit that it has been fairly painful for me.  It's not so much that I fear for his eternal salvation or anything like that - I think God is big enough to handle all of that and my friend's questioning and rejecting his faith.  It's more that I mourn the loss of relationship with him.  We weren't the best of friends; we knew each other through attending the same church.  But what (de-)conversion to atheism has meant for this person is no longer going to church and mostly abandoning his old Christian friends.  I know he has a least one good friend who also is an atheist, so perhaps that played a role in his (de-)conversion.

It strikes me that this guy is now an atheist in the way that I hope no one is a Christian.  He is disrespectful and dismissive of Christian beliefs and the idea of God.  Perhaps I read too much into it, but it strikes me as being disrespectful and dismissive to his Christian friends as well.  He is also somewhat evangelistic (though I'm not sure that it is "good news" that he has to share).  He hopes that others are also (de-)converted to atheism and is forming a club to promote atheist views.

In my less charitable moments, I wonder if this attitude is part and parcel with atheism.  It pretty much is by definition more about what you are against (theism) than what you are for.  It's in the name itself.  Christians, at least, have some positive content to their faith that they can share for the good of the world.  I'm sure that's the case with atheists too, but sometimes I wonder.  Maybe they can focus on promoting their particular views of epistemology, science, and reason, and less on demeaning the faiths of their neighbors.

One painful thing about my friend's (de-)conversion has been seeing similarities between the way he has acted and the way some Christians act when they are newly converted as well.  Does becoming a Christian (or atheist) mean abandoning your old friends, dismissing and belittling their beliefs, and aggressively trying to promote your religion (or lack thereof)?  I hope not.  I don't want to abandon the idea of evangelism, but if it looks like this, I'd rather that everyone kept their beliefs to themselves.