Thomas takes a scientific approach to religion. He doubts the witness of his coworkers and friends until he can see for himself. Jesus provides him with the opportunity. He then calls blessed those who accept the truth of the resurrection without seeing for themselves.
The Christian church lives or dies on the basis of believing the testimony of others. We have no records of the resurrected Lord appearing to non-believers. True Paul gets struck down on his way to Damascus. But that vision is in a significantly different category from the resurrection appearances. It is years later. Others with Paul see nothing.
Blessed are those who have not seen and yet have come to believe. Why?
Because the truly significant religious experiences are reserved for those who already have committed to the work. This is why a Quaker meeting for worship is different from sitting at home in your easy chair. There is an intention to meet the Christ spirit (or however you label that) in the silent waiting. The prayer, speak, for your servant is listening is the prior condition to hearing, not because God only speaks to the faithful, but because faith teaches us to know what it is we have heard.
Why is Thomas singled out for this object lesson? Is it because Thomas actually doubted and this is a record of that? Or was the spirituality of the gospel of Thomas already known by John and he saw it as a threat to his own community’s faith?
And for myself. Like the Twin I want certitude before I step out in faith. John is telling me through Thomas that I must step into the darkness before I will see the light. I fear falling.