This is an answer to a question Toni wrote when she was a ghostwriter for gotquestions.org

Question 35999: Unmarried Couples Living Together

            First, be very clear on one point – What you told your friend is not close-minded or simply your opinion, but the very Word of God. “You have heard that it was said, ‘You shall not commit adultery”. (Matthew 5:27) Premarital sex falls into the same category. Any sex outside of marriage is against God’s will.

On the other hand, Matt 7:1 tells us, “Stop judging others and you will not be judged.” Also, the bible tells us in Romans 14:1 to accept Christians who are weaker in their faith and don’t argue with them about what they think is right or wrong. Additionally, gentleness is always more persuasive than accusation. Be sure that your motivation is loving and not self-righteous. Romans 14:4 says, “Who are you to condemn God’s servants? They are responsible to the Lord so let Him tell them whether they are right or wrong.” But, they were talking about what pleases God and we know that breaking the Lord’s commandments is not pleasing to Him.

This is tough because we do not know if your friend is a believer behaving badly, (John 3:36), or a nonbeliever behaving normally according to secular standards. “The man without the Spirit does not accept the things that come from the Spirit of God, for they are foolishness to him, and he cannot understand them, because they are spiritually discerned.” I Corinthians 2:14

So if she is a nonbeliever – Romans 3:3 says, “What if some did not believe {and} were without faith? Does their lack of faith {and} their faithlessness nullify {and} make ineffective {and} void the faithfulness of God {and} His fidelity to His Word?” Of course not! As Christians, we know that premarital sex is sin. But unless we are dealing with another believer, it will fall on deaf ears to tell her that. Instead, pray for her whenever you think of her and let God turn her heart and show her His purpose for her life.

You could do little else than tell your friend that she is living in sin. Now, just show her Christ in your lifestyle and pray, continuing to love her, asking for God’s blessing on her life and a gradual turning of her heart for her eternal sake.