The one who is a friend of Jesus will be the target of the enemies of Jesus (15:18-25). By being a friend of Jesus, the disciples also inherit his enemies! This too is a function of a friend in the ancient world, to be a friend of one person was to also gain enemies.
Perhaps an analogy can be drawn to modern politics, where taking a particular political position can align you with a friendly party, but make you the target of another. It is possible that someone might align themselves with a particular candidate and create a tension with old friends who find that candidate offensive. (Imagine Ted Nugent coming out in support of Ted Kennedy. That probably would not end well for either of them!)
Jesus makes it clear to his disciples identification with Jesus is to accept hatred from the world, and that hatred will develop into a very real persecution. This description of persecution returns to the metaphor at the beginning of the chapter. The friends of Jesus will be attacked by the world, but this is to be accepted as “pruning” from God.
The disciples of Jesus can be expected to face persecution because of their association with Jesus. In fact, it is the claim that Jesus, and only Jesus, is the Lord which opens the disciples up to persecution.
For a Jewish audience, to call someone Lord seems to violate the foundational principles of the Hebrew Bible as found in the shema. If the Lord is One, and Jesus is Lord, then he must be the One God. While that has been the claim of John’s gospel all along, that is a shocking statement for the Jewish world to accept. We know from the book of Acts that there was a suppression of the apostolic claim that Jesus is Lord.
For a Roman audience, to call someone Lord seems to violate the foundational principles of the Roman empire. Domitian, the emperor who ruled when John wrote his gospel, identified himself as Lord and God, Nero called himself as the Savior of the World, and even Augustus was described as the divine son who brought peace to the world. Since Christians were making these same claims about Jesus, they were in danger of persecution from Rome for impiety and treason.
Jesus does not promise his disciples a comfortable life, free from trouble. He does not promise them good health, a fat bank account and a mega-church in Texas. He promises them that if they are really his friends, and if they are bearing witness to his words, and if they are really keeping his commandment to love one another, the world will hate them.