We observe what happens to bodies (i.e. falling, why we can stand 'upside down" on earth and not 'fall" off, etc). We can also measure the force. It is detectable, consistent and it is universal. We just don't know why it exists. But you are free to doubt and jump off a cliff. That's why there are not that many doubters of gravity today—they're all at the bottom of some cliff.
I think that kosta50 will make an Evangelist yet.
All except for the “there are not that many doubters” part,
that was a good description of God's Word. Post 2,765