Really? Because I'm pretty sure I'm not a Christian.

This narrative that atheists and agnostics are just christians that chose not to believe in God to be able to sin is far to common, and super inaccurate. I have (multiple) atheist friends who were once evangelical Christians with no plans to commit sin, in fact their life plan was to be an apologist. Ever since leaving Christianity non of them have done or have planned to do anything that would widely be considered immoral. (Most now consider their previous plans involving very manipulative techniques to be very unethical) Most atheists that have left Christianity did so because they found it increasingly hard to convince themselves (and the threat of hell is a very good reason to keep believing) that an old book and it's contents, that frequently contradict experimental observation, should be the center of their life.