St. Thomas fell into the sin of doubting because he had fallen into the sin of pride. He was tempted to pride not because he was weak but because he was strong. The temptations stemming from our strengths are probably the most dangerous.
Thomas was smarter than the average apostle. When Jesus went to the dead Lazarus, He risked His life. Thomas understood this and made the great profession of faith: "Let us go along, to die with Him" (Jn 11:16). Thomas was intelligent enough to ask Jesus the question which Jesus answered with one of the greatest statements ever made: "I am the Way, and the Truth, and the Life" (Jn 14:6). Thomas may have been absent from the upper room on the evening of the day of Jesus' resurrection because he was not so bound up with fear as were the other apostles (see Jn 20:19, 24, 26).
When the weak, fearful disciples gave Thomas the message of Jesus' resurrection, Thomas was tempted to be proud and to reject what they said. Thomas was too smart for his own good. He gave in to intellectual and spiritual pride. He sinned.
However, Thomas did not stay too proud to repent. He admitted his sins and his strengths, professed Jesus as his Lord and God (Jn 20:28), received the Holy Spirit at the first Christian Pentecost (Acts 2:4), and became a missionary, martyr, and part of the foundation of the Church (Eph 2:20).
Admit your sins and strengths. Repent and be Thomas' twin (see Jn 20:24).