The conversion of Thomas may have been one of Jesus' greatest miracles — not because Thomas was so weak but because he was so strong. Weak people are almost forced to conclude that they need help — even God's help, while strong people are severely tempted to rely on themselves. In our weakness, God's power can reach perfection (2 Cor 12:9). In our strength, God's power may seem unnecessary.
Thomas was probably a strong person. He was absent when the risen Jesus came to the other apostles, who were locked in a room because of fear (Jn 20:24, 19). This means Thomas was not locked up. Also, when Jesus went to Bethany to raise Lazarus from the dead, Thomas boldly encouraged the other apostles: "Let us go along, to die with Him" (Jn 11:16). Thomas was probably more courageous, free, and fearless than the other apostles.
Thomas' conversion was a great miracle — not because Thomas was so simple-minded but because he was so intelligent. Simple-minded people find it hard to keep up the pretense that they know what they're doing. Intelligent people may conclude that what they understand is sufficient. Thus, much of God's plan is "hidden from the learned and the clever" (Lk 10:21).
Thomas was probably an intelligent man. He had enough sense to ask the insightful question which elicited Jesus' claim to be "the Way, and the Truth, and the Life" (Jn 14:6). Even when Thomas spoke of probing Jesus' wounds (Jn 20:25), he was likely advocating what we call the "scientific method" of collecting data.
Jesus converted Thomas despite Thomas' proclivity to fall into temptations to self-reliance and intellectual pride. When Jesus converted Thomas, He showed He can convert anyone. There is hope for all to be saved (1 Tm 2:4).