John 20:29
Then Jesus told him, “Because you have seen me, you have believed; blessed are those who have not seen and yet have believed.”
Why is it that those who have not seen are blessed? Because those who have not seen are resting in faith. Faith is the only response that man can muster from his free will that acknowledges God as the only source of good, unconditional love and truth, while simultaneously laying down his (man's) own assertions to the same.
Thomas believed because he passed what he observed through his own intellect deciding for himself that the evidence supported the claim of the resurrection. In this, he acted as judge, placing the Creator on trial.
For those who believed without seeing, they submitted wholly to the truth of the news without needing to see physical evidence. This is a picture of complete submission to the Lordship of Christ.
Had Adam and Eve simply believed God when he said that eating of the tree of the knowledge of good and evil would bring about their death, they too would have remained "in the blessing". Instead, they like Thomas, decided that they needed to pursue for themselves their own understanding of truth through experience. Thus a turning away from God, and unto self. The ultimate denial of the creator (sin).
Those who did not see yet believed are blessed because they have learned to die to self and have allowed Christ to be formed in them. They are living by the fruits of the spirit. Thank God for us all, for that which was lost in unbelief and selfish pride, can be restored by Grace alone, through Faith alone, in Christ alone!