We as Christians are all called to preach the gospel, and you may be faithful in doing that. But have you ever considered whether a message of salvation by works has crept in without you realising it?
You know that salvation is a free gift. You know it's not earned, but can only be received by faith alone in the sacrificial death of Jesus. And this is the message you preach. Perhaps you even quote Ephesians 2:8-9 which states very helpfully and clearly that it's by grace we are saved through faith and not of works. But despite that, when speaking with an unbeliever about his response required to the gospel, do you unintentionally share a message of salvation by works?
Now what do I mean?
You're chatting with an unbeliever, you've talked about the cross and his need of salvation, and now you are up to explaining the response required, and so you say this:
"There are two things you must do to be saved: Turn from your sins and trust in Jesus."
You might say, what is wrong with that?
The issue is that the unbeliever has just interpreted your statement as "stop sinning and trust in Jesus" or "be obedient from now on and trust in Jesus".
Now that is not what you've intended for them to hear. When you said "turn from your sins" maybe you meant - have a change of mind about your sins which will lead to a change of action), but the normal and most common interpretation of being told to "turn from your sins" is: "stop doing those sins, be obedient, be a good person." You can do a survey of a bunch of unbelievers and that is what they'll undoubtedly say.
So essentially this is what you have unintentionally preached (because the person has misinterpreted what you said):
OBEDIENCE + FAITH => SALVATION - INCORRECT!
The great truth of Christianity is that salvation is a free gift, no works are required to receive it (Romans 3:24-26). Faith alone is required (sola fide). Romans 5:1 spells this out well: "Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ."
Since we are justified by faith alone, our aim in communicating the gospel to unbelievers should be for them to understand this. Naturally, the non-Christian will think that being a good person or improving his life or stopping his sins is what will get him to heaven. The gospel message teaches the opposite.
If obedience is not required to receive salvation, where does obedience come in then? Faith will necessarily lead to obedience. But obedience is the fruit of salvation, not one of the means of having it. How someone lives after making a profession of faith shows whether the faith he has is real or dead. James 2 talks about real faith verses dead faith. True faith produces works as a fruit (obedience being one of those good works).
To be clear, if someone continues living any way he wants, that shows he doesn't actually believe. Obedience evidences the person's faith, it doesn't cause it.
So what we should want the person to understand is this:
FAITH => SALVATION + OBEDIENCE - CORRECT!
How would you explain this then? There are many ways you could communicate it, but here is an example of one way:
"You cannot earn forgiveness by improving or stopping your sin. Instead trust that the sacrifice of Jesus has paid the penalty for all your sins. That means, no longer rely on your goodness to save you, but only in what Jesus has done to save you. And this will cause you then to start living your life for Jesus has done out of thankfulness for what he has done. You will no longer want to live your life your own way."
It is wonderful that you go and share the gospel with the lost, but we should be careful that what we say doesn't get misinterpreted by the unbeliever. We may know clearly what we mean, but they may hear it as something else. Taking this on board will help each of us become better witnesses for Christ!