Mark Chapter 15

Mk.15:31"He saved others," they said, "but he can't save himself!
Mk.15:32Let this Christ,^[b] this King of Israel, come down now from the cross, that we may see and believe."

Here the chief priests and the teachers of the law mocked him. I think it's quite interesting to read, that they said "that we may see and believe."(v.32).
With this sentence they express their unbelief. Because if you see and then believe, then you don't have to believe anymore, because you have already seen it. That's not how faith is.
Faith is "the substance of things hoped for, the evidence of things not seen."(Hebrews11,1... I hope you all know this verse very well!!).
So as we have seen something, then we cannot say "I believe, that it exists", because by seeing it we already know, that it does exist.
We can also look at thomas' example:

John20:27 Then He said to Thomas, “Reach your finger here, and look at My hands; and reach your hand here, and put it into My side. Do not be unbelieving, but believing.”
John20:28 And Thomas answered and said to Him, “My Lord and my God!”
John20:29 Jesus said to him, “Thomas,^[d] because you have seen Me, you have believed. Blessed are those who have not seen and yet have believed.”(John20:27-29)