If your goal is to simply counter them and let them know there are happy, informed nonbelievers in the world, then that is easy. Just be yourself, say what you think, and don’t worry if they change their minds. Be relaxed about it. You are not the one with the problem and you can’t solve everyone else’s problems. If you try too hard to change their mind, it can make you look uneasy, like you are “protesting too much,” which they most certainly will take as a sign of insecurity. It can also validate their “war” (as the hymn says, “Onward, Christian Soldiers”), hardening their resolve.
If your goal, however, is to convert them, then you have a lot of work to do. Well, actually, we can’t “convert” anyone. We all have to come to our own conclusions. If you were raised religious, like me, you know that your de-conversion came from inside, not from an atheist evangelist. But still, some of us atheists and agnostics do feel a need, or responsibility, to champion reason, science, and kindness, and would like to improve the world by persuading others to abandon superstition and dogma.
Taken from this article.