Last week I answered a question from a woman who isn't sure how to turn down a date with an unbeliever. She wrote,

A few weeks ago a non-Christian asked me out. I didn't know he was interested in me, was taken off guard, and didn't know how to respond. I felt very uncomfortable saying, "Sorry, I won't spend time with you because you aren't a Christian." Instead, I made up a lame excuse and left. I'm sure that was not the best way to handle the situation. It certainly wasn't honest. What do you think is the most sensitive and loving way to explain that you only date other Christians?

It's a good question both because it reaffirms the biblical command to only date and marry believers, but also because it reminds us that some ways of saying no are better than others. I replied,

In order to follow through on your desire to date and marry a believer, you must love Christ above all else. He must be more valuable to you than any human relationship — He must be your greatest treasure. Then, rooted in His love and empowered by the Holy Spirit, you will not only have the ability to obey His commands, you will also be able to do so — including saying no to a date with an unbeliever — in ways that testify to your faith.

So what to do in this specific situation you’ve raised? In short, you should reply “No, thank you.” Said with a smile, those three little words are powerful for guarding you from all sorts of undesirable situations. ... sometimes it’s necessary to say more than that. I agree it’s not a good idea to say, “Sorry, I won't spend time with you because you aren't a Christian.” But you can certainly say, “As a Christian, I’m committed to dating only those who share my faith.” A response like this may cause a young man to stop and think, and may open his eyes to the Gospel.

You can read my full answer at Boundless.org.