My mother tells people, "I believe none of what I read and only half of what I see." She is, of course, engaging in hyperbole, but you can understand the intent. On one hand, people lie, people are mistaken, people are confused. They tell me that eyewitness accounts of catastrophic events are often twisted by the eyewitness's own emotional response, so it isn't always accurate. We know that people will tell us things with a straight face that they know are not true because they want to believe it. And, of course, others will repeat it as if it is true because, well, they heard it. Or most of us have heard, "It must be true; I read it on the Internet", which is a joke based on the fact that so many people seem to take nonsense spewed on the Internet as absolute fact. Then there's the whole problem of "seeing is believing". Given Photoshop and CGI and all that can be done to manipulate images these days, what can we really believe of what we see? Probably my mother's hyperbole is too far, but it is really undeniable that any thinking person should treat with some skepticism most of what they read and even see.
So when it comes to faith, you can understand that believing might be somewhat difficult. I mean, we're asking people to believe in an all-powerful being they cannot see and take as true words they cannot hear. We present to them a Savior they've never known who ... get this ... died, rose again, and now lives in heaven. Oh, and in our hearts. Yeah, that ought to fly. Ask many skeptics, "What would it take to convince you?" and what you'll get is precisely a form of "seeing is believing". "If your Jesus was to stand here in front of me right now and talk to me, I'd believe." Or something like it.
You'd think that made sense. You'd think that it was reasonable that God should show people Himself, His Son, His reality. He could heal and He could walk among us and He could demonstrate His power and who, then, could deny it? You'd think that, but you'd be wrong. That is a materialist view, based on the world around us, not reality. Because, you see, "The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned." (1 Cor 2:14) That is, we're not dealing with normal, everyday stuff. We're dealing with spiritual warfare, with issues of which "the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God." (2 Cor 4:4)
I'm not making this stuff up. Look at the biblical evidence. Beyond what I've just given you, consider. In John 10 the crowd demanded, "If you are the Christ, tell us plainly." Jesus told the crowds, "I told you, and you do not believe. The works that I do in My Father's name bear witness about Me." (John 10:25) He fed crowds with a few fish and loaves, healed blind and lame people, raised the dead, and more. When Jesus raised Lazarus from the dead, He did it in their presence (John 11:32-44). The skeptics that were there ran off to tell the skeptics that were not (John 11:45-46). Their response was not belief. They determined they had to kill Jesus and Lazarus both (John 11:53; John 12:9-10). Elsewhere Jesus told the story of the Rich Man and Lazarus (different Lazarus) (Luke 16:19-31). The rich man, in torment, begged that someone could be sent back from the dead to warn his brothers. He is told, "If they do not hear Moses and the Prophets, neither will they be convinced if someone should rise from the dead." (Luke 16:31)
How about a Christmas example? Zechariah (Luke 1:5-20) was a priest married to Elizabeth, a relative of Mary, who was barren. While Zechariah was doing his priestly duties in the temple, an angel -- the archangel Gabriel, in fact -- appeared to tell him that Elizabeth would bear a child. Well, now, that is seeing, right? An angel from heaven shows up to tell him what is about to happen. How did that work out? "And behold, you shall be silent and unable to speak until the day when these things take place, because you did not believe my words, which will be fulfilled in their proper time." (Luke 1:20) Seeing without believing.
We'd like to think that people are rational beings. We tend to believe that if only they saw enough evidence, they'd come to realize the truth. We prefer to conclude that seeing is believing, and, whether they physically or simply mentally see, they will be convinced. But seeing is not believing in all cases. It is certainly not the deciding criterion when it comes to spiritual faith.
When the skeptics in Jesus' day refused to believe even when Jesus was doing undeniable miracles in their presence, He explained, "I told you, and you do not believe. The works that I do in My Father's name bear witness about Me, but you do not believe because you are not of My sheep." (John 10:25-26) Apparently, then, the problems of the deceitful heart (Jer 17:9) and spiritual blindness (1 Cor 2:14; 2 Cor 4:4), need to be overcome before believing occurs. And that's not accomplished by seeing. This is why the Lord must open hearts (e.g., Acts 16:14). In terms of saving faith, despite all our bold claims to the contrary, seeing is not believing. In this case, it takes a miracle, a miracle we all pray for in our own lives and the lives of those we love. A miracle, as it turns out, God is quite good at.