"Seeing is believing. If I can’t see it, I don’t believe it exists."
We believe in many things that we can’t see. (See Optical Illusions tract) Ask a skeptic if he has ever seen the wind. Has he seen history? Has he ever seen his brain? We see the effects of the wind, but the wind is invisible. We have records of history, but it is by "faith" that we believe certain historical events happened. Television waves are invisible, but an antenna and a receiver can detect their presence. The unregenerate man likewise has a "receiver." However, the receiver (his spirit) is dead because of sin (Ephesians 2:1). He needs to be plugged into the life of God; then he will come alive and be aware of the invisible spiritual realm.