"Religious" to me refers to organized Christian churches, their members, beliefs and teachings, rather than any other religious affirmation, such as Muslim and Judaism. I think it is because Christianity is far and away the predominant religion not only in the USA, but also amongst most other Western world populations. It's what I am familiar, and grew up with. Most people around me are Christian.
Also, while admittedly a completely nonpracticing Christian, I am somewhat familiar with the fundamental principals of Christianity (eg God, Jesus, the Bible, the commandments, sin, etc). Though I lack any meaningful knowledge of, say, Judaism (even though many of my closest friends are Jewish), their beliefs are far enough detached from my understanding of Christianity that they cease to be thought of as practicing a religion. I have a difficult time accepting a non-Christian person as religious, for instance, if Jesus is not believed to be fundamentally important to that religion.
I must be setting Christianity as a sort of "gold standard", inadvertantly. If other non-Christian beliefs don't come close to this standard, they apparently cease to be considered a religion by me.
Good question.








