The gospel rightly preached contains a “built-in” apologetic. In other words, when we get the story of redemption right, human boasting and glory are eliminated, while the stage is simultaneously set so that all the glory goes to the Father for His sovereign plan, to the Son for His righteous life and perfect sacrifice, and to the Holy Spirit for His gracious drawing. But, never to us.
How does this help us in defending and advancing the gospel?
Detractor: How do I know that you are telling me the truth?
Answer from the Gospel: If I had told you that I was such a good person that God had to forgive me, accept me, and let me into heaven one day, then you would have reason to be skeptical. Such words make me look like a pretty good person, so perhaps I would have something to gain by telling you these things. However, that’s not what I told you. I told you that I was such a miserable failure before God, a law-breaker, hostile to Him, with absolutely nothing good to offer Him, that Jesus had to die, spilling his blood on the cross, as my substitute so that I could go free.
Not only does this sort of answer disarm at least the questioner’s suspicion about your motives, but it also gives an opportunity to once again clarify the gospel to this individual. It may not solve all of his intellectual questions, but it does speak to his heart. Remember, humanity’s primary problem is not intellectual; it’s moral.