Doubt.

“St. Thomas asks for the very thing the other disciples received […] he wants to see the risen Christ for himself. St. Thomas shows us that doubt, even deep doubt, is not the opposite of faith. Doubt need not be a sign that faith is weak and dying. Doubt can be a sign that faith is alive, longing for the risen Lord to renew our life. Perhaps precisely on account of his doubt, Thomas is able to make the strongest confession of faith of any of the disciples in the gospels: ‘My Lord and my God.'”
– Fr. Kevin McGrath, O.S.B