"I have made myself a slave to everyone, to win as many as possible. To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. To those not having the law I became like one not having the law (though I am not free from God’s law but am under Christ’s law), so as to win those not having the law. To the weak I became weak, to win the weak. I have become all things to all people so that by all possible means I might save some. I do all this for the sake of the gospel." 1 Corinthians 9:19-23
"Do not love the world or anything in the world. If anyone loves the world, love for the Father is not in them. For everything in the world—the lust of the flesh, the lust of the eyes, and the pride of life—comes not from the Father but from the world. The world and its desires pass away, but whoever does the will of God lives forever." 1 John 2:15-17.
Is there a tension between these verses? The passage in 1 Corinthians seems to be telling us to blend in with the world in order to win them for Christ whereas 1 John appears to tell us to maintain a separation.
In reality, there is no tension. Notice that 1 John does not say to completely cut yourself off from the world but rather not to "love" the things of the world (such as lust and pride). Even Jesus became part of the world so that he could experience what tempts us ("For we do not have a high priest who is unable to empathize with our weaknesses, but we have one who has been tempted in every way, just as we are—yet he did not sin"; Hebrews 4:15). When we say Christians are to be "in the world, not of the world," that does not mean that you must segregate yourself from your surrounding culture. In order to properly reach people where they are, you must understand that culture, but be disciplined enough to pull back if you start to be tempted by "the lust of the flesh, the lust of the eyes, and the pride of life."
Be the seed that fell on good soil (Luke 8:8, 15). If you know that your heart lies with God, then you can "become all things to all people" without "loving the world." God bless.