We're supposed to believe the Word simply because God said so! The Lord wants us to have faith in His Word and not always have to have a sign. The Word is all the sign we need!
Why does God insist we have to believe in something we cannot see and cannot feel, purely by faith in His Word, trusting Him just like a little child has to trust a parent? It's because the Lord loves faith! He loves us because we believe Him, just because He said so! It's a way of showing our love and our confidence in Him. Faith really shows that we have love.--"But without faith it is impossible to please Him: for he that cometh to God must believe that He is, and that He is a rewarder of them that diligently seek Him." (Heb.11:6)
It's just like a child has to trust a parent even though he doesn't always understand why he must do or must not do this or that. He just has to "do it because Daddy says so!" It's a form of love that the child trusts his parents and has confidence in them and takes their word for it.--And that's the way we have to do with the Lord. We should just say to Him, "Yes, Sir!"--And believe it and do it simply because He said so!
Trust Him! Take Him at His Word!