On the evening of that day, the first day of the week, the doors being shut where the disciples were, for fear of the Jews, Jesus came and stood among them and said to them, “Peace be with you.” When he had said this, he showed them his hands and his side. Then the disciples were glad when they saw the Lord. Jesus said to them again, “Peace be with you. As the Father has sent me, even so I send you.” And when he had said this, he breathed on them, and said to them, “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained.” Now Thomas, one of the twelve, called the Twin, was not with them when Jesus came. So the other disciples told him, “We have seen the Lord.” But he said to them, “Unless I see in his hands the print of the nails, and place my finger in the mark of the nails, and place my hand in his side, I will not believe.” Eight days later, his disciples were again in the house, and Thomas was with them. The doors were shut, but Jesus came and stood among them, and said, “Peace be with you.” Then he said to Thomas, “Put your finger here, and see my hands; and put out your hand, and place it in my side; do not be faithless, but believing.”Thomas answered him, “My Lord and my God!” Jesus said to him, “Have you believed because you have seen me? Blessed are those who have not seen and yet believe.” Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.
The “blessed doubt” of the Apostle Thomas. St. John of Karpathos wrote:
“‘ Blessed are those who have not seen, and yet believed’ (John 20:29). Blessed also are those who, when grace is withdrawn, find no consolation in themselves, but only continuing tribulation and thick darkness, and yet do not despair; but, strengthened by faith, they endure courageously, convinced that they do indeed see Him who is invisible.” (St.John of Karpathos in The Philokalia Volume 1 – The Complete Text, pg. 315)
From the ancient Church we also find this on doubt:
“There are some who, even though they have begun to develop a taste for divine things, nevertheless are disturbed and hassled by the adversary, so that they are surprised (still lacking experience) that after the divine visitation, they should still harbor doubts about the mysteries of the Christian religion. Those who have grown old in them are not surprised at all. As skilled farmers from long experience, if they have had a year of bountiful harvest, they do not live without some planning, but they foresee the time of dearth and tight times.
On the contrary, if famine and penury hit them, they do not become despondent, as they think positively about the future. It is the same way with things in the spiritual world. When the soul falls into various temptations (Jb 1:1), it is not surprised, not does it lose all hope, because it knows that by God’s permission it is being exposed to trials and is being disciplined by evil. Nor does it forget other circumstances when things go well and there is consolation, but it expects the time of trial.
The sun, being a material, created thing, shines down also upon swampy places full of mud and slime and yet is not affected or defiled. How much more the pure and Holy Spirit that is joined to the soul which still is afflicted by evil, without himself being tainted by the evil. For: ‘The light shines in darkness and the darkness comprehended it not’ (Jn 1:5).” (Psuedo-Macarius, The Fifty Spiritual Homilies and the Great Letter, pg.130)