It sounds to me you have been talking to what they like to call themselves as "anti-theists"
If they call themselves anti -theists you know they are going to attack you regardless of what you say. I have however spoken to some atheists that were interested in having an intellectual debate.
I have to say, I never approach an atheist with the gospel, I let them come to me for the debate. I hate to spark a controversy, it's a waste of time, and could deter them further away from the gospel which is not good.
Most atheists on internet forums are there to attack, and it can get very childish, although saying that, I've noticed christian forums aren't that much better ! >.<
Good luck with your ministry
Keep up with the good work!