In the hours before His arrest, trial, and execution, Christ celebrated the Passover with His closest friends—the disciples. In the intimacy of the upper room, He encouraged them with promises of the powerful, Spirit-enabled ministry awaiting them, pointed ahead to their eternal home in heaven, and spoke warmly of His love for them.
But the conversation wasn’t all good news. In John 15:17-25, Jesus issued a sober warning to the men who would extend His ministry to the ends of the earth.
This I command you, that you love one another. If the world hates you, you know that it has hated Me before it hated you. If you were of the world, the world would love its own; but because you are not of the world, but I chose you out of the world, because of this the world hates you. Remember the word that I said to you, “A slave is not greater than his master.” If they persecuted Me, they will also persecute you; if they kept My word, they will keep yours also. But all these things they will do to you for My name’s sake, because they do not know the One who sent Me. If I had not come and spoken to them, they would not have sin, but now they have no excuse for their sin. He who hates Me hates My Father also. If I had not done among them the works which no one else did, they would not have sin; but now they have both seen and hated Me and My Father as well. But they have done this to fulfill the word that is written in their Law, “They hated Me without a cause.”
In his book The Upper Room, John MacArthur explains the reason for Christ’s warning.
But in addition to all those promises, Christ needed to warn His closest friends. They needed to know that, in spite of the wonderful divine promises that would be fulfilled in their experience, life would not be completely blissful. Ministry would not be easy in a rebellious, Christ-hating world. The world was going to treat them the same way it treated Him. They were going to be despised and persecuted—even killed.
Verse 17 is Jesus’ transition from describing His love for the disciples to describing the world’s hatred: “These things I command you, so that you will love one another.” The Greek verb indicates a continuous action: “Keep on loving each other,” He is saying. “Devote yourselves to one another and sacrifice for one another. Love each other the way I loved you.” That’s a simple summary of everything He had been saying all evening. . . .
One reason their love for each other was so important was that the world would know nothing but hatred for them. Love for each other was the only love they would ever know. In a hostile world, they desperately needed love from each other.  John MacArthur, The Upper Room (The Woodlands, TX: Kress Biblical Resources, 2014) 195-196.
Jesus was highlighting the fact that the disciples would spend the rest of their lives as aliens and strangers in the world (1 Peter 2:11). In short, they—and every generation of believers who followed them—were going to be in the world, but not of it.
That phrase is familiar to most Christians. Paraphrased from Christ’s words of warning to His disciples, it has become a slogan for God’s people, and an encouragement when the world feels particularly foreign and hostile. It highlights the natural friction we feel with the world around us.
In The Upper Room, John MacArthur explains the source of that constant friction.
People living in the world who do not know Jesus Christ are part of a system that is anti-God, anti-Christ, and satanic. That system militates against God and His principles. It is opposed to all that is good, godly, and Christlike. I am always amazed at the way some Christians seem to believe the world can easily be persuaded to admire Jesus if we try to portray Him as a stylish superstar to be idolized. That failed (and still failing) strategy is one of the major reasons persecution is on the rise. In any contest for the world’s affections, the truth will always be marginalized. “The light has come into the world, and people loved the darkness rather than the light because their works were evil” (John 3:19). No unbeliever will ever truly embrace Christ apart from the Holy Spirit’s convicting and regenerating work. The church’s duty is to preach the Word of God and proclaim the gospel—even in the face of the world’s hostility.  The Upper Room, 199-200.
The entire world system—every government, philosophy, movement, and trend—are pitched in direct opposition to the Lord and His Word. Some are more blatant about it than others, but every aspect of life in this world that has not been redeemed by God is working in contradiction to Him.
No wonder we feel out of place. As John MacArthur says, “Authentic believers in Christ simply cannot fit into the world’s system. We are supposed to be different. We have different values, a different Lord, and a completely different agenda.”  The Upper Room, 198.
In that sense, the friction between the world and the church is not simply natural, it’s necessary.
In the days ahead, we’re going to consider the implications of being in the world, but not of it. We’ll look at the world’s ongoing campaign against the truth, and how our friction with the world serves as a testimony of God’s transforming work that ought to convict and compel other lost sinners. And we’ll consider practical ways to live apart from the influence of the world without stifling our testimony to it.