No, not because it was written down. However, you know that it's very wrong, and you know that the police will come and get you. Basically, even though you may not know that there will be repercussions in the next life or whatever, you know that there will be repercussions. And you do it anyways. So an atheist can commit mortal sin, i suppose. it seems odd to me, though. It's like someone who didn't get a warning getting in the same trouble as someone who did get a warning. now, yes, the atheists did get the same warning, but maybe God didn't give them the same reason to believe it as He gave us. So, now, would everything that would be considered a mortal sin for us be the same for atheists? Peace!