Do you believe in the gravity? We've all heard the old saying "what goes up must come down" but you can't see it so how do you know it exists? You know because you experience it every moment of every day. The http://www.yourdictionary.com/ definition is : grav·i·ty [ grv-t ]n. Physics a. The natural force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, tending to draw them toward the center of the body.
If someone told me they belonged to a group that didn't believe in gravity and to prove to the world it wasn't real they were going to jump off a cliff, I would plead with them not to do it, I would remind them that other people who jumped off cliffs did so with tragic consequences, I would even call the authorities to get them to prevent the person from jumping. I would do all this because I believe in the law of gravity even though I can't see gravity. I believe it because I experience it.
To other Religions of the world Christians are an infuriating lot. We don't just believe in God, His Son, Jesus Christ, and the Holy Spirit, we know They are real because we've experienced Their reality. I've had prayers answered down to details. I get directions and instructions from God in my Spirit and from His Word. Many tragedies have been averted in my life because I've placed my life in God's care. We can't be "politically correct" because the bible we read says that salvation is only available to us through Jesus Christ. Frankly there's just not enough time left to tiptoe around the subject trying not to offend anyone. We honestly don't want to offend anyone, we just don't want them to jump off the cliff.