I always thought atheists didn't believe in Jesus or his teachings. I was wrong. I found this interesting website that tells another side of what atheists may believe: http://www.atheists-for-jesus.com
It appears that they don't necessarily believe in Jesus, per se, as much as they believe in his alleged message, as indicated in the intro to that webpage: Welcome to Atheists for Jesus! A Site designed to provide a method of communication between religious and non-religious people who believe in the message of love and kindness put forth by Jesus. I have to disagree that Jesus' main message was about love and kindness - that was merely the sheep's clothing it was dressed in. Here are a couple of webpages that help to reveal the kind of person Jesus really was (even if he was just a fictional character, as I actually believe to be the case): Things We Didn't Find Out in Sunday School Jesus: The Truth If Jesus' message had actually been about love and kindness, then I would be in full agreement with those Atheists. But since they are Atheists, I suspect they really haven't read much of the bible themselves, and that they are just going by what they've heard from Christians regarding Jesus' message - a huge mistake, in my opinion.
Any atheist can believe that Jesus existed, but not that he was actually the son of God. I have always claimed agnostic to save me the trouble of having to form a defensible position on the subject, so I think it's possible that Jesus existed and possible that he was the son of God.