1 Nephi 3

Alma commented to the poor of the Zoramites -

"...there are many who do say: If thou wilt show unto us a sign from heaven, then we shall know of a surety; then we shall believe." (Book of Mormon | Alma 32:17)

But he reality is that is not so among a rebellious and disobedient people. The Savior the people of His day -

"...What sign shewest thou then, that we may see, and believe thee? what dost thou work?
31 Our fathers did eat manna in the desert; as it is written, He gave them bread from heaven to eat.
32 Then Jesus said unto them, Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven.
33 For the bread of God is he which cometh down from heaven, and giveth life unto the world.
34 Then said they unto him, Lord, evermore give us this bread.
35 And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.
36 But I said unto you, That ye also have seen me, and believe not." (New Testament | John 6:30 - 36)

There is something about the rebellious in that in their iniquity without sincere repentance, they receive not of the Spirit and prevents them from belief. This was never more evident as Nephi says that as Laman and Lemuel -

"...as they smote us with a rod, behold, an angel of the Lord came and stood before them, and he spake unto them, saying: Why do ye smite your younger brother with a rod? Know ye not that the Lord hath chosen him to be a ruler over you, and this because of your iniquities?" (Book of Mormon | 1 Nephi 3:29)

It is apparent that Laman and Lemuel were full of inquity. If the appearance of an angel is not a sign, I don't know what is. The angel even proceeded to promise -

"...Behold ye shall go up to Jerusalem again, and the Lord will deliver Laban into your hands." (Book of Mormon | 1 Nephi 3:29)

After witnessing such a marvelous sign, and after the angel departed -

"Laman and Lemuel again began to murmur, saying: How is it possible that the Lord will deliver Laban into our hands? Behold, he is a mighty man, and he can command fifty, yea, even he can slay fifty; then why not us?" (Book of Mormon | 1 Nephi 3:31)

It becomes clear that even though there are many that say that if a sign is shown, then they will believe, is obviously false. That is why -

"...these signs shall follow [only]them that believe..."
(New Testament | Mark 16:17)