4 And now behold, when they saw this they were astonished exceedingly, insomuch that they fell to the earth; for they had not believed the words which Nephi had spoken concerning the chief judge.
5 But now, when they saw they believed, and fear came upon them lest all the judgments which Nephi had spoken should come upon the people; therefore they did quake, and had fallen to the earth.
We don't have to see to believe. Paul says of faith, that it is "the substance of things hoped for, the evidence of things not seen." (Heb. 11:1) To have faith, we need only perceive the truth through the power of the Holy Ghost. As we exercise faith in the teachings of Jesus Christ and apply them in our lives, evidence is heaped up to strengthen and reward our faith.
For example, there is a command to search the scriptures. If I act in faith to follow that commandment, I will hear His voice and know His words. (D&C 18:34-36) As I hear the voice of the Lord in His word, my faith in Him will increase and I will receive that peace which passeth all understanding. (Philip. 4:7) And in this spiral of increasing faith and testimony I will come to know for myself that the scriptures are true and that God lives. All of this comes from the application of faith in things that I cannot initially see.