Korihor, the Anti-Christ, reasons that if you cannot see it, it must not be real, therefore there is no such thing as a Christ, and that to believe such is the effect of a frenzied mind. From Alma 30:15-16
How do ye know of their surety? Behold, ye cannot know of things which ye do not see; therefore ye cannot know that there shall be a Christ.
Ye look forward and say that ye see a remission of your sins. But behold, it is the effect of a frenzied mind; and this derangement of your minds comes because of the traditions of your fathers, which lead you away into a belief of things which are not so.
How do you counter such lies? Here’s the Prophet Alma’s response from Alma 30:44:
But Alma said unto him: Thou hast had signs enough; will ye tempt your God? Will ye say, Show unto me a sign, when ye have the testimony of all these thy brethren, and also all the holy prophets? The scriptures are laid before thee, yea, and all things denote there is a God; yea, even the earth, and all things that are upon the face of it, yea, and its motion, yea, and also all the planets which move in their regular form do witness that there is a Supreme Creator.
I can’t think of a better response.
Source: Alma 30:16-16, 44; The Book of Mormon: Another Testament of Jesus Christ