It’s sad to watch someone you helped lead to Christ turn back to their old ways. It makes you wonder if their conversion was genuine or if they became like the prodigal son that Jesus mentioned in the gospels who eventually turned around and came back. Whatever the case is, you cannot live their lives for them, but you can count on the promise that he will not lose even one. What then do we do? Rest assured that if the Holy Spirit drew them in, it would be he that brings them back. Don’t become overly concerned, but reach out to God and intercede for them, and leave the outcome to him. Ultimately, it is God who chooses; you are only the vessel he works through. God’s word will fall into place, and the truth will be revealed to them and brought into the light for them to see. You will then see a change of heart; they will be happy to return and ashamed that they ever turned away from him. Wait for it; it will happen.
For this reason, when I could bear it no longer, I sent to learn about your faith, for fear that somehow the tempter had tempted you and our labor would be in vain. 1 Thessalonians 3:5