Doubt is not the same as unbelief. Doubt is uncertainty. It's that murky area between belief and unbelief where nothing is decided, and nothing is committed to.
Thomas wasn't afraid to ask questions when he didn't agree or understand something. Jesus never put him down for it or anyone with an honest doubt or question. For such a person is seeking to believe. They want integrity in their faith.

To be without any questions or doubts at all is to block us off from the blessings of growth and the discovery of truth and ideas. Thomas had questions. He asked them because he wanted to understand. Doubt, to Thomas, was simply faith seeking understanding.