If it’s true that seeing is believing, I wonder what some of the things might be that you believe because you have seen them with your own eyes. Maybe you were not sure, but once you have actually seen them, you are convinced. The Pyramids, perhaps … the Taj Mahal … Angkor Wat …
At a more mundane level, perhaps a child tells you that they have tidied their room and you think “A likely story!” But when you check, they have! Seeing really is believing.
Jesus has already appeared to Mary, but - as a woman in those times - she is not considered a reliable witness. Now the disciples have an appearance of their own; so it is true after all. For them, too, seeing is believing. But Thomas is not there and he is sceptical when he hears their story. He wants a slice of the action; he needs to see Jesus for himself.
Graciously, Jesus appears again and Thomas’ need for tangible proof is dispelled. Amongst other things, I think that Thomas’ experience is meant to be an encouragement to us. We are not eye-witnesses (born too late!) but we can depend on the testimony and witness of those who were there.
Thomas is the first person in John’s gospel to look Jesus in the eye and say: “My Lord and my God!”
John’s primary aim is to point each one of us towards Jesus, as the Son of God. Has he succeeded, or has he wasted his time?