*Edited July 10, 2015*
The purpose of witnessing the Gospel is not to convince people. Rather, it's informative — to inform people about the Good News of Jesus Christ. Think of a witness on the stand in a courtroom: canceling out perjury, he doesn't give his account to convince people what happened, but to let them know what he knows. If they don't believe it, that's not his problem. So when you witness and people don't believe the Good News that you tell them, that's not your fault and it's not your problem. When we hear the Word of God, we are responsible for it — responsible for believing it or rejecting it. St. Paul said, "...I would not have come to know sin except through the Law; for I would not have known about coveting if the Law had not said, 'You shall not covet' " (Romans 7:7). What such true words these are! The only reason we know what it means to sin is because the Law in God's Word tells us what sin is, and the vitality of spreading the Gospel of Jesus Christ is to make known the free forgiveness of sins through God's gift of grace.
We are all sick in sin, and how can we know we're sick if somebody doesn't diagnose it and then tell us about the remedy? Jesus said, "It is not those who are healthy who need a physician, but those who are sick... for I did not come to call the righteous, but sinners" (Matthew 9:12, 13b). Adding to this, Morton Kelsey said, "The church is not a museum for saints but a hospital for sinners." We don't go to the doctor when we're healthy, but when we're sick. Think back to your childhood or more recent times in your life when you've refused to go to the doctor when you've been sick or hurt. Times when you've said, "I don't wanna go to the doctor. I don't wanna take that medication." It doesn't matter if this was during your adulthood or childhood. We have all refused to accept sickness and take its remedy in the physical sense at some point in our lives. In the same way, we do this with our spiritual sickness as well. We are all sick sinners, and there are many who refuse to accept the fact that they're sick in sin (which is diagnosed with the Law) and then refuse to believe and take the remedy for sin (forgiveness through the grace of Christ — the Good News/Gospel, which are the same word in Greek, εὐαγγέλιον). When we Christians go out to make this remedy known (the Gospel), we are not the doctors — Jesus Christ is the Doctor (yay Doctor Who reference!). Through witnessing, it is not our duty to force the remedy upon them. It is our duty to offer them the remedy through the diagnosis of the Law, and it is up to them whether or not to accept the diagnosis and take the remedy given to them. If they refuse, there is nothing we can do about it.
So, as we all go out into the world to tell of the Good News of Jesus Christ, I exhort you all to refrain from debates and arguments with those who reject the truth. When we go out to preach the Good News, we must have the fruits of the Spirit within us (Galatians 5:22-23). When we preach in condemnation or persuasion tactics, we are not practising these fruits of love, joy, peace, kindness, goodness, faithfulness, meekness, and self-control. The purpose of arguing and debating is the attempt to force someone to accept our truth, but that is both impossible and unchristian. It is unchristian because God has not called us to debate and argue with others in animosity. And it is impossible because such people who debate and argue against us have already made up their minds; they are not looking to be convinced out of their current position and they therefore cannot be moved because they're perfectly comfortable where they are. No one wants to move when they're comfortable. That's why it's always hard for me to get up off the couch.
I exhort you all to just simply make known the Gospel as statements of fact, and if anyone chooses to debate against you in a hostile manner, withdraw from their presence; nothing good will come of it. Engaging in their hostile behaviour will not lead them to repentance, but will only draw them further away from the Lord. And do not be weary; you have not failed. Rather, you have planted a seed. St. Paul teaches that Christian teachers plant and water, but it is God who causes the growth (see 1 Corinthians 3:5-7).
