Thomas refused to believe what He couldn't see or feel. Our five senses were given to us by God and are necessary to help us function in this life. But if we do not renew our minds to know the limits of the five senses, they will keep us from believing. Faith can perceive things that the senses cannot (Hebrews 11:1).
The type of faith that Thomas had was a human or natural faith that was based on what he could see. Jesus said there was a greater blessing to be obtained. That greater blessing comes from using a supernatural, God-kind of faith that is based only on God's Word.
Jesus and God's Word are one (John 1:1, 14). Believing God's Word is not just putting your trust in some printed words on the pages of a book we call the Bible. There is much more involved. It is a relationship with a person, the person behind the words. We get to know Him through His Word.