From a comment by a Jewish blogger: “Judaism isn't a religion in the way that Christianity, and only Christianity, is. The word ‘religion’ comes to us through the filter of Christianity, but the collection of entities it refers to have almost nothing universally in common.”
From a recommended post on Tumblr: “Some conservatives openly believe that ‘freedom of religion’ only means ‘freedom to choose a Christian denomination,’ and many more quietly believe it. They also believe that only Christianity really counts. Like, recently Reason argued that Judaism isn't really a religion & thus can't claim ethical stances like defending abortion, since it's not Christian enough in structure. They literally don't see those other religions as religions at all!”
Sometimes it’s just a matter of what valence you use.