One important aspect in evangelism is the use of reason. This is something that God did with His people of Israel through the Prophet Isaiah. “Come now, let us reason together, says the Lord: though your sins are like scarlet, they shall be as white as snow; though they are red like crimson,they shall become like wool”(Isaiah 1:18 ESV). God is reasonable and He invites us to reason with Him. His desired outcome in reasoning with us is that our sins would be washed away.

This is also something that the Apostle Paul did in his evangelistic strategy.
“And as he reasoned about righteousness and self-control and the coming judgment, Felix was alarmed and said, “Go away for the present. When I get an opportunity I will summon you” (Acts 24:25 ESV). The three things that Paul specifically reasoned with Felix about were righteousness, self-control, and the coming judgment. It appears that Felix was feeling convicted of his sin and wasn’t ready to repent so he told Paul to “Go away for the present”. Reasoning with people who don’t know Christ about righteous, self-control and judgment can be the means that the Holy Spirit uses to convict and draw someone to Christ.
If you are going to reason with a person about righteousness, then the 10 Commandments are the essence of what to use for reasoning. The commandments show us what is right and what is wrong. I do this personally with those who don’t recognize their need to be saved from their sin. I learned it from Ray Comfort’s teaching titled Hell’s Best Kept Secret. It’s really simple to do but it is not comfortable for either parties. Talking about sin usually isn’t. What I do usually do is ask them if they have ever lied, stolen or lusted. Then I ask them what that makes them and if God were to judge them by that standard would they be innocent or guilty. Would they go to Heaven or Hell? All this can help prepare someone to see the good news of the gospel. For so many the good news makes no sense because they don’t understand the bad news.

Watch these two videos to see examples: