How Do We Live in the World But Not of the World?

I am struggling with a friend who professes to be Christian, yet does not believe in the Trinity or that Jesus is Lord. If we are to be “in the world but not of the world,” then should I stop being friends with her? But if I do, she may not have others who will talk to her about spiritual matters.

All Christians are called to be "in the world" but not "of the world:"

John 17:14 “I have given them Your word; and the world has hated them, because they are not of the world, even as I am not of the world.
John 17:15 “I do not ask You to take them out of the world, but to keep them from the evil one.
John 17:16 “They are not of the world, even as I am not of the world.

Being "of this world" means following the unbelieving world's values, beliefs, and conduct. Christians are not to share in these things. Instead, we are to follow Christ's word and reflect it by our words and actions.

On the other hand, Jesus specifically says the Church was to remain in the world for a time. While we are here, the Christian is an ambassador for Christ, Paul says.

2Cor. 5:20 Therefore, we are ambassadors for Christ, as though God were making an appeal through us; we beg you on behalf of Christ, be reconciled to God.

Just as our nation's ambassadors are commissioned to live on foreign soil for a time so they can represent our nation in that place, so are Christians called to live for a time in a world that is not our eternal home so we can represent Christ's kingdom to unbelievers.

Furthermore, Christ expects us to represent Him in a loving way. We are not sent to act as judges and critics (as the Pharisees were) nor are we seeking to shame unbelievers for their sins. Rather, we know it is the kindness of God that brings men to repentance, and so we come to represent the Father's love and kindness in the face of Christ. Like any good ambassador, we want to represent the mercy and grace of our King by reflecting the light of Christ, and in so doing, we hope to attract the unbeliever to Christ.

Jesus lived this example for us in the Gospels. Jesus never sinned, yet He made Himself available to sinners on a regular basis. He ate with sinners, the Gospels tell us. He counseled prostitutes. He stayed in the homes of tax collectors and other evil men. He was willing to associate Himself with sinners in this way, because Jesus was sent to heal the sick, and as He said, healthy people don't need the services of a doctor.

Similarly, Christians aren't called to restrict our associations to only Christians or sinless people - if this were the case, we could have no friends at all! Instead, we are called to live in and among the unbelieving world as witnesses to Christ. To do this, we must associate ourselves with unbelievers and immoral men. In fact, Paul commands us to do this very thing:

1Cor. 5:9 I wrote you in my letter not to associate with immoral people;
1Cor. 5:10 I did not at all mean with the immoral people of this world, or with the covetous and swindlers, or with idolaters, for then you would have to go out of the world.
1Cor. 5:11 But actually, I wrote to you not to associate with any so-called brother if he is an immoral person, or covetous, or an idolater, or a reviler, or a drunkard, or a swindler — not even to eat with such a one.

Ironically, Paul says the only immoral people we are NOT supposed to associate with are immoral Christians! On the other hand, we are supposed to live among immoral unbelievers, for there is no other way to exist in this fallen world. In fact, Paul says the only way to keep yourself entirely free from from immoral people would be to leave the world altogether. Clearly, this is not our calling.

So in your situation, you are free to spend time around your unbelieving friends, including those who think they are Christian but follow false Gospels. Fellowship without expressing judgment or seeking to cause them shame. As you speak with them, be a witness for Christ and the true Gospel. You are Christ's representative and perhaps He will use you to open their eyes to the truth.