Jesus' apostles and disciples had more experiences of the miraculous than anyone else in the world, either before them or after. They saw Jesus do the supernatural on a daily basis, from feeding thousands with a few loaves and fish, to healing the sick, to raising the dead to walking on water in the midst of a powerful storm. He also told them many, many times that He was God, but still many of them did not believe.
What finally clinched it for Thomas, who many people still call "Doubting Thomas" 2,000 years later, was when, after being tortured, abused, flogged 39 times, hung on a cross and pierced in the heart with a lance, Jesus came back from the dead, just as He had promised them all along He would do:
24 But one of the Twelve, Thomas (called “Twin”), was not with them when Jesus came.
25 So the other disciples kept telling him, “We have seen the Lord!”
But he said to them, “If I don’t see the mark of the nails in His hands, put my finger into the mark of the nails, and put my hand into His side, I will never believe!”
26 After eight days His disciples were indoors again, and Thomas was with them. Even though the doors were locked, Jesus came and stood among them. He said, “Peace to you!”
27 Then He said to Thomas, “Put your finger here and observe My hands. Reach out your hand and put it into My side. Don’t be an unbeliever, but a believer.”
28 Thomas responded to Him, “My Lord and my God!”
29 Jesus said, “Because you have seen Me, you have believed. Those who believe without seeing are blessed.” John 20
Transliterating from the Greek, what Thomas said was,
"You are the Lord of me and the God of me!"
Where Does the Bible Say That Jesus is God?