It takes a lot of courage to declare in daily life that one is an Atheist. It can adversely influence job prospects, long held friendships and more. The anonymity of the internet allows atheists free rein to express their feelings, and so it's a popular pastime.
There are of course those who just come here to poke fun at sincere believers. To brand them as atheists would frankly be an insult to conscientious atheists. These people are just rather immature, and making fun of Christians gives them an unwarranted feeling of power. Just ignore them.
I myself am an atheist. So why do I come here? Well, just because I'm an atheist does not stop me wanting to find out more about what other people think, and I am interested about what makes people believe in religion. I try always to be constructive in my criticism and to respect the views of others. I have learned a lot from people here. And yes, the words of some Christians have helped me to modify some of my views. So stick with it. You may be doing more good for your cause than you realize!