From The Candle of the Lord by Elder Boyd K. Packer, of the Quorum of the Twelve Apostles:
I will tell you of an experience I had before I was a General Authority which affected me profoundly. I sat on a plane next to a professed atheist who pressed his disbelief in God so urgently that I bore my testimony to him. “You are wrong,” I said, “there is a God. I know He lives!”
He protested, “You don’t know. Nobody knows that! You can’t know it!” When I would not yield, the atheist, who was an attorney, asked perhaps the ultimate question on the subject of testimony. “All right,” he said in a sneering, condescending way, “you say you know. Tell me how you know.”
When I attempted to answer, even though I held advanced academic degrees, I was helpless to communicate.
When I used the words Spirit and witness, the atheist responded, “I don’t know what you are talking about.” The words prayer, discernment, and faith, were equally meaningless to him. “You see,” he said, “you don’t really know. If you did, you would be able to tell me how you know.”
I felt, perhaps, that I had borne my testimony to him unwisely and was at a loss as to what to do. Then came the experience! Something came into my mind. And I mention here a statement of the Prophet Joseph Smith: “A person may profit by noticing the first intimation of the spirit of revelation; for instance, when you feel pure intelligence flowing into you, it may give you sudden strokes of ideas … and thus by learning the Spirit of God and understanding it, you may grow into the principle of revelation, until you become perfect in Christ Jesus.” (Teachings of the Prophet Joseph Smith, comp. Joseph Fielding Smith, Salt Lake City: Deseret Book Co., 1977, p. 151.)
Such an idea came into my mind and I said to the atheist, “Let me ask if you know what salt tastes like.”
“Of course I do,” was his reply.
“When did you taste salt last?”
“I just had dinner on the plane.”
“You just think you know what salt tastes like,” I said.
He insisted, “I know what salt tastes like as well as I know anything.”
“If I gave you a cup of salt and a cup of sugar and let you taste them both, could you tell the salt from the sugar?”
“Now you are getting juvenile,” was his reply. “Of course I could tell the difference. I know what salt tastes like. It is an everyday experience—I know it as well as I know anything.”
“Then,” I said, “assuming that I have never tasted salt, explain to me just what it tastes like.”
After some thought, he ventured, “Well-I-uh, it is not sweet and it is not sour.”
“You’ve told me what it isn’t, not what it is.”
After several attempts, of course, he could not do it. He could not convey, in words alone, so ordinary an experience as tasting salt. I bore testimony to him once again and said, “I know there is a God. You ridiculed that testimony and said that if I did know, I would be able to tell you exactly how I know. My friend, spiritually speaking, I have tasted salt. I am no more able to convey to you in words how this knowledge has come than you are to tell me what salt tastes like. But I say to you again, there is a God! He does live! And just because you don’t know, don’t try to tell me that I don’t know, for I do!”