The best way to witness to an agnostic, an atheist, or a mere skeptic is to remember that salvation is of the Lord (Jonah 2:9). The Christian does not need to debate the unbeliever, but simply declare the gospel. “That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ” (1 John 1:3).
The gospel which is declared by a Christian is then revealed to the unbeliever whom God will save by the power of the Holy Spirit. Paul wrote about this Divine act of mercy and grace which he personally experienced. “But when it pleased God, who separated me from my mother’s womb, and called me by his grace, 16 To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood” (Gal. 1:15-16).
In addition to declaring the gospel, and relying upon the Holy Spirit, a believer can live such a Christ like life that those around you see a difference.
Dr. H. A. Ironside was once walking up Market Street in San Francisco on a Sunday, and the Salvation Army was holding a meeting at the junction of Market Street and Grant Avenue. The captain, recognizing Dr. Ironside, asked him to give a message, and he gladly agreed. After the address a well-dressed gentleman stepped up to Dr. Ironside and handed him a card on which he had been writing. On one side was his name, Arthur Morrow Lewis, the well-known agnostic lecturer. On the other side he had written: `Sir, I challenge you to debate with me the question, “Agnosticism versus Christianity”, in the Academy of Sciences Hall next Sunday afternoon at 4 p.m. I will pay all expenses.’
Dr. Ironside read the card aloud and replied, ‘Mr. Lewis, I already have an engagement for next Sunday at 3 o’clock, but, if necessary, I think I could cancel it. I am disposed to accept your challenge and will if it is really worthwhile. But in order to prove that you have something worth debating, I accept on these conditions: First, that you promise to bring with you to the platform next Sunday one man who was once an outcast, a slave to sinful habits, but who on some occasion heard you or some other infidel lecture on agnosticism, and was so helped by it that he cast away his sins, became a new man, and is today a respected member of society, all
because of unbelief. Second, that you will also agree to bring with you one woman who was once lost to all purity and goodness, an abandoned female sunk in the depths of depravity, but who can now testify that agnosticism came to her while deep down in sin and implanted a new hatred of impurity in her poor heart, putting a new power into her life and delivering her from her base desires, and making her now a clean, chaste woman, all through disbelieving in God and the Bible. Now, sir, if you will agree to these conditions, I will promise to be there with one hundred men and women who were once just such lost souls as I have described but who heard the precious gospel of the grace of God, who believed it and ever since have hated sin and loved righteousness and have found new life and joy in Christ Jesus, the Savior Whom you deny. Will you accept my terms?”
He shook his head and turned away while the crowd applauded. They knew that in all the annals of agnosticism no one ever heard of unbelief making bad people good, but the Bible has demonstrated its power in untold myriads of cases to turn men from sin to righteousness, from darkness to light and from the power of Satan to God. (Acts 26. 18) 20.
If the opportunity come for a rational conversion, the Christian can point out that it makes more sense to believe in God than to question the existence of God or believe that time, plus space, plus chance brought into existence a universe that reflects an Intelligent Designer.
Neither the universe, nor man, is a cosmic accident. Man is not a grown-up germ which emerged from a primordial soup. His will, intelligence, and emotions reflect that he was made in the image of God.