"Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed" (John 20:29).
Jesus was willing to give doubting Thomas the visible evidence he wanted before he would believe. However, He did give His disciple a mild rebuke.
There is an important principle here. Thomas was willing to believe, but only when the visible evidence was too strong to question. Neither the promise of Christ that He would rise from the dead nor the testimony of His chosen apostles that the promise had been fulfilled was sufficient to convince him, and the Lord was disappointed.
When God has spoken plainly in His word, that ought to be sufficient for those who really believe Him. Yet again and again Christians allow their faith to be shaken by some new cosmic theory, or age estimate, or something else. No matter how strong the Biblical case for the worldwide Flood may be, for example, many Christians will not believe it until all the geological questions can be resolved. Even though the Bible unequivocally teaches that all things were created in six literal days (see Exodus 20:11), many Christians won't accept this until they can see overwhelming scientific evidence of a young Earth. In fact, some will never believe in either recent creation or a worldwide Flood until all the secular scientists accept them first.
God has allowed many visible evidences of the truth of His word to be revealed. There is a strong scientific case for Biblical creation, and we are justified in believing God's word, even where we don't yet see any visible evidence. As Peter said concerning those who believe implicitly in Christ and His word: "Whom having not seen, ye love; in whom, though now ye see Him not, yet believing, ye rejoice with joy unspeakable and full of glory" (I Peter 1:8). HMM