Most people have been blessed with the gift of sight. We can see objects around us, and show those objects to each other. With most objects, sight is not the only way to prove they exist. I don't have to show you my phone. I can make it play some music, I can put it in your hand, you can even sniff or lick it if you're a complete weirdo. If you lose your sight, I can prove to you that I have a phone.
But what about the sun? The sun, by its nature, can be detected only by one sense: Sight. You can see the sun, but you have no other sense by which you can distinguish the sun from other objects. The distance of the sun, combined with our inability to survive being close enough to it for any length of time, means our vision is the only sense by which we can directly observe the sun.
So, how do we prove the sun exists to a man who was born without vision? Men born blind cannot even conceive of what it is like to see. In fact, recent advances in medicine show that if you give a blind man a simple ball, then give him surgery so he can receive his sight, he could not identify the ball just by sight. He would have to touch it again. Basically, blind men do not have access to the one sense with which we can prove the sun exists. They must accept it on faith.
Now let's imagine a world in which only two people have sight. The rest of the human race is completely blind. How would the two sighted people prove the sun exists?
Well, they could appeal to the heat of the sun. But of course, even a blind man knows that there are other sources of heat, some of which they can touch. They don't need to believe in a big, burning ball of gas more than 90 million miles away to believe in heat. You can explain anything away if you try hard enough.
The most logical step, then, is to first prove that you can, in fact, see. If you can see, it at least makes sense that you would know if the sun exists, and so it would be quite reasonable for a blind man to trust that yes, the sun exists, because you, who can see, say it exists.
Now, is it possible that things exist which humanity as a whole cannot sense? The obvious answer is yes, especially since we now have equipment that can sense it. Using this equipment, we also find that some animals can sense it, too. Some animals have night vision. Some animals can detect UV rays or heat signatures. Some animals use the earth's magnetic field to navigate and orientate. Though we weren't all born blind, all humans have limited perception of the real world.
So, is it at least possible that a spiritual realm exists that cannot be detected by science? Only a fool would say no. Of course it is possible that a spirit realm exists that we cannot prove by conventional, scientific means. So the next questions are does such a realm exist, and how might we prove it? The answer: Yes, it exists, and we prove it as a blind man proves the sun.
As a blind man accepts the sun exists on the testimony of a sighted person, we can accept the testimony of "sighted" people that God exists. Christianity has never been a religion of blind faith. Contrary to the popular myth, God isn't just a fictional character who was made up beside a campfire during the bronze age. Rather, God proved His prophets can "see" by giving them powers no man can access apart from Him. Each new message God revealed was surrounded by a series of miracles. Actions performed by men that defy human explanations.
Jesus is the prime example. During His life, He performed many miracles we could not replicate, even with our modern technology. Oh sure, a skilled magician might be able to turn water into wine (or at least make it appear so), but how many magicians can cure genetic blindness? How many people can tell a paralytic man "get up and walk" and have him obey? Not even a doctor can take a 3 day old corpse and raise him. Let's be honest: If a modern doctor could replicate even one of Jesus' more famous medical miracles, he'd become a national sensation over night, and revolutionize the medical industry. This is not the kind of skill one would expect from a first century carpenter.
And yet, not only did Jesus do all of these things, He was also executed publicly, and after that, seen alive and well publicly. Even if a modern doctor could raise the dead, could he raise himself from the dead? Certainly not. And yet Jesus did.
So, I'd say Jesus has proven His "sight" beyond reasonable doubt. Is it not therefore reasonable to say that yes, when He says the "sun" (God, the spirit world, Heaven, Hell, all of that) exists, the "sun" exists? Put it this way: I would have a far greater amount of respect for a blind man who doubts the existence of the sun than for an atheist who, having examined the historical evidence for the Gospel, continues to say there is no God.