Religious posters in the workplace?

People commonly post personal (not company related) invites to various functions in a break room - baby showers, get-togethers, religious functions. I don't really see a problem with an invitation, so long as it's not some kind of condemnation or it's not something company sponsored. I think it's possible you are being overly sensitive due to feeling lonely, as you described, during this time of year. If you have a good relationship with the folks in your office, host a small holiday get together that is festive and not religious. You might have fun and feel better.