I learned this in my journey toward fatherhood. Babies, for the first "while" after they are born cannot focus or see much of anything unless you're really close to their face. You have this little person (or little people in my case) that can't see and her only way to communicate with us big people is to make a noise. Not to mention, it's the only noise she can make right now. There's variations to the noise, but it's all basically a type of crying out.
The point is that the world, to newborns, is extremely fuzzy. They can see variations and stuff...but they do not see the physical world like we see it. Their eyes need to be trained.
John 20:24-29
"Now Thomas, called the Twin, one of the twelve, was not with them when Jesus came. The other disciples therefore said to him, 'We have seen the Lord.' So he said to them, 'Unless I see in His hands the print of the nails, and put my finger into the print of the nails, and put my hand into His side, I will not believe.' And after eight days His disciples were again inside, and Thomas with them. Jesus came, the doors being shut, and stood in the midst, and said, 'Peace to you!' Then He said to Thomas, 'Reach your finger here, and look at my hands; and reach your hand here, and put it into My side. Do not be unbelieving, but believing.' And Thomas answered and said to Him, 'My Lord and my God!' Jesus said to him, 'Thomas, because you have seen Me, you have believed. Blessed are those who have not seen and yet have believed.'"
Thomas needed to see and touch in order to believe. Thomas wanted to use the physical senses he had to convince himself of the impossible. Something had happened that was impossible. I think sometimes we are hard on Thomas here. People came and told Thomas that Jesus had risen from the dead; not an easy sell. Of course, if a group of people corroborate the story - - it's harder to not go along with it. Even still, I think Thomas shows us that no matter how overwhelming the stories are toward proving the impossible true, we will still seek some kind of proof before we believe it. Honestly, I kind of applaud Thomas in part for his ability to stand his ground and not believe. It had to take some kind of courage to tell all the other disciples that he wasn't going along with this nonsense that Jesus was alive. Right or wrong - he defended what he thought to be the truth. Don't be a hypocrite and judge Thomas so quickly, we do this all the time.
For a second time, Jesus hits the scene and greets the disciples. He goes right to Thomas and they go through proving that Jesus is who He says He is. Thomas now believes because it has been proven to him to be true according to the standards that He created in His own heart. Jesus gave Him what He needed to believe. Like the awesome savior He is Jesus follows this up with a brief teaching statement, "Thomas, you have believed because you have seen. Blessed are those who have not seen and yet believed."
Which brings me to an interesting point. 2 things that sometimes seem to stand in direct contradiction to one another. Defending the truth and believing. I don't know that I've ever seen a human being defend a truth they claimed not to understand. I've seen them defend things they think they understand mostly, or are trying desperately to understand - but not something that is hopeless for them to understand. That's the trick of it. We cannot hope to understand how it is that Jesus stood in the midst of the disciples; how it is that He rose to be seated at the right hand of the Father; how it is that He sent the Holy Spirit to us; any more than we can understand the ways of God. We believe these things by faith, because (I would argue) we cannot be shown these things. The separateness of God is so great that if He would describe to us how it worked I think we would lose Him at, "Well, you see it's like this..."
Let's stop back at the teaching statement again to finish up.
"Thomas, you have believed because you have seen. Blessed are those who have not seen, yet believed."
I see a hidden challenge in this verse to Thomas from Jesus. Very clearly, it is a message to Thomas that it is better to not need to see in order to believe. In other words, your faith should not be tied to something as physical as your own sight. That being said, it begs the question...what should your faith be tied to? How to we keep on believing?
I'd like to suggest that how we choose to see our world and the circumstances that occur within our field of view greatly shapes the state of our faith. For example, do you have faith that God is with you in a difficult circumstance in your life; or is it that when God responds to your difficult situation you will praise Him because He clearly (in hindsight) is worthy? It is a subtle difference in principle, but a great chasm in practice. Your attitude through a difficult time will reflect whether your faith is tossed to and fro like a weed blowing in the wind, or whether it is founded on the cornerstone that is Christ, not shaken by all that you see - because you are assured of what is unseen.
Believe.
