When you talk about Christianity with an unbeliever, the discussion depends on one main issue: authority. “Why should I believe what you say about spiritual matters?” a skeptic might ask. You must be ready for this issue by asking yourself some questions: Is the gospel true merely because you’re telling it to them? Is it true merely because you heard it all your life? Or is the gospel true because it comes from an all-knowing God who cannot lie?

But there’s the issue! Not everyone believes that the Bible comes from God. And when you’re talking with an unbeliever about spiritual things, they will often discount what you’re saying on this very ground: “You’re telling me things from the Bible, and I don’t accept the Bible.” Whether they tell you this or not, they’re probably thinking it.

How do you give the gospel to someone who rejects the fundamental presupposition of your whole life and worldview? You tell them why they too should believe that the Bible is the Word of God. You can do this in two basic ways: experientially or rationally.

Experientially means giving the gospel, while conscious of the fact that the unbeliever’s own experience validates many of its claims. Just give them the gospel, knowing that their own conscience will agree with a lot of what you say. This is powerful because it appeals to their own thoughts, which, no matter how deeply suppressed, agree with God’s Word. Which verses in the Bible agree with sinners’ consciences? Verses about sin, Christ’s Law, and judgment.

Press verses about sin. People know, deep down, that they are terrible sinners (Rom. 1:31). They have flashes of insight in which they see their hypocrisy in judging others when they do similarly evil things (Rom. 2:1). It is incredibly convincing to see the Bible deal honestly with us about ourselves. The Bible doesn’t whitewash our sin. It doesn’t flatter us. It condemns us for the cruel, gossiping, arrogant, limelight-loving, selfish beings that we know we are, if we’d be honest. This is incredibly convincing, because it shows people that the Bible is simply an honest book. Everyone else is trying to ignore or downplay sin. The Bible points it out, describes it, and condemns it. Read Romans 1-3. Illustrate verses on sin by sharing how convicted you were of your own sin when you got saved, and how you saw that your sin was heinous, not minor. Sin in the seed is essentially the same as in the full tree.

Press verses about Christ’s Law. Jesus’ teaching in the Sermon on the Mount nearly converted Mahatma Gandhi. Gandhi’s main reason for not becoming a Christian was that He didn’t like Christians. But the teaching of Jesus haunted him, and it’ll haunt others too. There is special convicting power in those words: “You’ve heard it said ‘don’t murder’ but I say ‘don’t be angry.” Anger unrestrained leads to murder. Jesus demands a clean heart, not mere outward conformity. Jesus wants a heart of love, which genuinely cares even for those who hurt us. “Love your enemies.” This is unparalleled; all our hearts long for it, and the only thing that keeps us from it is ourselves. I once worked with a Bosnian Muslim who fought in Sarajevo in the early 1990s. This man had killed people on the other side, and the experience marked his personality. He was a very angry man. On one occasion, at work, another co-worker mocked my Muslim friend, who then came back to his workstation next to mine. He turned to me, and his large eyes were bulging and red with anger. He whispered to me through his thick accent that he wanted to “kill that guy.” I had been trying to witness to him for a long time, and I saw a perfect opportunity to bring conviction into his life. Even as he stewed in his murderous rage, I told him Jesus’ words about loving your enemy and doing good to those who despise you. I told him that Jesus has the right view of what we ought to be, not Allah. My friend’s eyes widened in shock at what I had told him, and he said “it is impossible to be that way.” But I saw the impact the words of Christ made upon him. No one can deny that the words of Christ call for a moral code that would bring unceasing peace on earth. It is the greatest moral code ever given, and it is crushingly convicting to see how far we fall short of it. Use Jesus’ Words to point out our need for what He did later: die for our sins.

It is reasonable to expect that the God who understands us so well also is distressed by our sin. Therefore, press verses that teach judgment. “The wrath of God is revealed from heaven against all ungodliness and unrighteousness of men” (Rom. 1:18). Explain that God is holy, sin is His arch-enemy, and He can tolerate none of it. It’s like poison to Him. He’s zealously pursuing a sin-free universe. “Cursed is everyone who continues not in all things which are written in the book of the Law” (Gal. 3:10). Illustrate the verses by referring to your own experience of conviction, when God was leading you to Himself. Remember the feeling of impending doom you had when you understood that you were being bad? Remember the feeling that the axe could fall at any moment? Remember your brokenness when you realized the pain you had caused others? Press upon them thoughts of judgment. It will resonate, though they may rebuff you.

Humanity is hard-hearted, but some people will hear the voice of their master in the verses you share. “My sheep hear my voice and they follow me” (John 10). Be “always ready” to support the gospel message by statements from the Bible that our own sinful hearts will agree to and, if the Spirit moves, bow to.