If your manager's recent change of heart was due to your comments at lunch, you might be facing religious discrimination. Many employees don't realize that religious discrimination protects believers and nonbelievers alike. Just as an employer may not refuse to hire a Seventh Day Adventist, promote only Buddhists, or target Muslim employees for layoffs, an employer may not treat employees poorly because they don't hold religious beliefs. It might seem odd to include atheism as a religious belief, but the law does. Just as employers may not treat religious employees poorly on that basis, they also may not favor religious employees over employees who are not religious.

But that answers only part of your question. Your manager's actions and statements are discriminatory only if they are, in fact, based on your comments. The timing and change of heart are facts that work in your favor: Shortly after you revealed your lack of religious belief, your manager downgraded your performance. However, you would need to show, among other things, that your performance really was as good as you thought it was before the lunchtime conversation, in the manager's eyes and in general. This can be tough for an employee to prove, given that someone else is responsible for judging the quality of your work.

On the other hand, if your manager can show that he has had long-term reservations about your performance, documented prior to your fateful lunch, that would work against you. The way the manager has treated other employees who are not religious might also come into play. If, for example, you can show that nonreligious employees are inevitably eased out of the company, your argument is stronger. If, on the other hand, the company can point to managers and long-term employees who are openly not religious, your argument is weaker.

The ultimate question isn't whether you could prove discrimination in court, however: It's what you want to do about your current predicament. You've had a performance review that was worse than you expected, along with an ominous comment, but you still have your job. It makes sense to start in the conference room rather than the court room. Invite your manager to meet one-on-one and express your concerns. Indicate that you were surprised by the performance review, that you want to stay at the company, and you're concerned that your recent conversation about religion may have affected your working relationship. Then wait to see how your manager responds. Make sure to get details about exactly how your performance has suffered and what your manager wants from you by way of improvement.

This conversation should tell you a lot about what really happened. If your manager can point to specific problems in your performance and steps you can take to turn things around, maybe your lunch conversation didn't change anything. But if your manager can't come up with much to say about your performance, and hems and haws when asked whether religion played a role in the evaluation process, it starts to look more like you may be facing discrimination. In that case, you may wish to bring your concerns to the HR department or a higher level manager (hopefully, one who is more open to differing religious beliefs).