Interacting with people who for various reasons haven't understood or believed the gospel of the finished work of Christ is not always necessarily a black and white process. I think the Apostle Paul showed that in his various dealing with others. Not that we always look to Paul and his ways of handling confrontation and debate as "the" ways for us to handle it, as he had his own personality and his own calling from God, but I for sure can say that I've gleaned a lot from him.
When speaking with others who are not walking in the freedom of the gospel, do you nip it in the bud quickly or do you take time to reason things out? Do you flush it out or do you flesh it out? I think it all depends upon getting a feel for where the other person is coming from.
At times Paul spoke harshly to and about those who rejected the gospel by putting any sort of confidence in the flesh - in their own fleshly attempts to become justified or remain justified. He would trample underfoot those who "trampled the Son of God underfoot." He flushed them down the toilet. He didn't really hold back at all. "Beware of dogs!" he said in Philippians 3. "Beware of evil workers, beware of the mutilation!" Those definitely aren't "tame" words. He had serious words for those who put any confidence in their own actions.
Read on in Philippians 3 and see how Paul said that he would have every reason to put confidence in his own actions, but yet he chucked it all aside and he counted it all as loss and traded it in "for the excellence of the knowledge of Christ Jesus my Lord... and be found in Him, not having my own righteousness, which is from the law, but that which is through faith in Christ..." He, of all people, would be the world's number one contender for putting confidence in his own deeds, but he knew that in order to have Christ he had to renounce it all. Therefore he had very harsh words for those dogs, those evil workers, those mutilators of the flesh!
On the other hand, there are those who he took time with. Instead of flushing them down the toilet, he fleshed it out with them. I see a form of the phrase, "he reasoned with them," at least seven times in the book of Acts. For example, "...and for three Sabbaths reasoned with them from the Scriptures, explaining and demonstrating that the Christ had to suffer and rise again from the dead, and saying, 'This Jesus whom I preach to you is the Christ.'" (Acts 17:2-3). "And he went into the synagogue and spoke boldly for three months, reasoning and persuading concerning the things of the kingdom of God." (Acts 19:8).
In these seven instances (and of course not only in these seven instances), there were people who heard Paul out and were persuaded and joined with Paul and the other believers, or at least wanted to hear more, and there were people who flatly rejected what he had to say. But at times, "some were hardened and did not believe," and Paul and his disciples withdrew from them. At times, people started mobs in reaction to Paul's "reasoning," and Paul and his friends ended up getting brought before rulers and thrown out of cities. Other times Paul stayed with the people for weeks, months or even a couple of years.
I said at the beginning that we don't necessarily follow Paul and all of his ways of dealing with those who hear and receive or reject the gospel. I think that to try to be a copycat of Paul is a huge mistake. The point here is that we can gauge where other people are at, and our response to them is not black and white. It often depends upon each given situation, and it's often a good thing to take time to get to know where someone is coming from rather than using a preconceived "speech" or way of handling things. Our own unique personalities play a big part as well. And most of all, obviously, we rely on the fact that we only go where God brings us and as we rest in Him we let His life in us do the confronting, persuading, reasoning, etc.