So I tell you this and insist on it in the Lord, that you no longer live as the Gentiles do, in the futility of their thinking.
Insist - maintain, vow, stand by it,
Gentiles - are considered the non believers of Chirst
Futility - Useless, pointless, senseless, ineffective, without lasting purpose, emptiness
Futility means there is a void of all spiritual matters and you don't care about your soul but only care about the flesh (new shoes, hair, clothes, latest fashions). Their eyes are blind to spiritual things, and remains in a depraved state.
People should be able to see a difference in you as a Christian that they did before you accepted Christ in your life. We should be full of light and joy. Remember the song "let yoru light shine, shine...
Paul told the Ephesians to leave behind the old life since they were now followers of Christ