When I was about 17, I moved congregations with my family and shortly thereafter their gospel meeting was upcoming. During that week a boy was invited and a few weeks later he obeyed the gospel. Honestly, over time I knew he was trouble, but I guess I just ignored it-- but then about a year later, to my parents dismay, we started dating. I was then a senior in high school and was about to move to college. When that time came, we decided to move to the same town and go to college together. We didn't live together but lived close by, and with no parents around there were no rules. We were caught up in sexual sin, and constantly sneaking around. We were always fighting and he was at points very violent. I basically was a huge hypocrite. I was going to services but I wasn't worshiping the Lord with my whole heart.
Eventually, my parents forced me to move home and he stayed. We eventually broke up and I got my life back together, repented of my sins -- both privately and publicly -- and started serving the Lord again. In that time, I met the love of my life and am now engaged, and will be married soon.
However, with my ex-boyfriend, he has stopped serving the Lord, and the congregation he attended is soon to withdraw from him as commanded in I Corinthians 5:9-13, and II Thessalonians 3:6. I guess wrote all of that to say I have felt so much guilt over this. If I as the "stronger" Christian would have stopped us from sinning, he would still be serving the Lord. My sweet fiance, who is also a member of the Lord's church, has been so sweet in all of this. He has told me it's not my fault, but I still care so deeply for his soul. It is nothing more than that -- I'm so in love with my fiance. Is that wrong, to still care so much about his soul, by all accounts of Scripture being lost? I have tried to contact him several times. Is there anything else that I can do for his soul? Am I still at fault for his soul being lost, even though I have repented for my sins?
"Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body" (I Corinthians 6:18).
Fornication is a bit different than other sins. While you are sinning with another person at the same time the sin you are committing is against yourself. In truth you are not sinning against the other person because he is a willing partner in the sin, though you can say that you are encouraging the other person to join you in sin.
Typically in such situations one person is the instigator and I gather that since you said you should have put a stop to it, I would assume that it began with him and you got caught up in it. It doesn't excuse what happened in any way, but I want you to realize that the idea of sinning was already in his mind before the sex started to take place. Add to it that you knew he was bad news -- which was a part of the attraction to him at that stage in your life and I can see that while he was converted, he never became grounded in the faith. He was among the seed that was choked by thorns (Matthew 13:22). Very likely, if it had not happened with you, it would have happened with someone else.
Your story illustrates what happens more often than not when sex enters the picture: the relationship stalls and then decays because everything revolves around the next opportunity to have sex. Yet eventually the excitement dies and each blames the other for the death, never realizing that this is the nature of sex. But because there is nothing else but sex holding the relationship together, everything falls apart.
I'm glad you do care about what happens to him. You did try to contact him. He, not you, chose not to answer. While it is a sad choice, it was his choice. He wasn't forced. Hopefully someone will get to him in the future, but for now your involvement with him is at an end. Your responsibilities have always been about your choices. You tried to reach out to turn him around. You cannot be responsible for his rejection of that offer. "The soul who sins shall die. The son shall not bear the guilt of the father, nor the father bear the guilt of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself" (Ezekiel 18:20).
It is time to close this chapter of your life. You made a lot of mistakes and learn a lot of lessons the hard way. You turned your life around and the Lord has blessed you with love. Focus on him and focus on continuing to improve yourself. "Brethren, I do not count myself to have apprehended; but one thing I do, forgetting those things which are behind and reaching forward to those things which are ahead, I press toward the goal for the prize of the upward call of God in Christ Jesus" (Philippians 3:13-14).