The Christian life is a holy life, a life set apart from the world. God says, “Go out from them and be seperate.” Our goal is not to be unnecessarily abnormal. And neither is it to seperate from sinners in every sense. Paul says that to leave off all dealings with sinners would require one to go out of the world.
Rather God would have us live in the world in a distinct way. Everything we do, we do in holiness. We live from him, through him, and to him. And the unbelieving world is living from self, through self, and to self. These two approaches to life are like oil and water, they simply do not mix. There they are, both in the bucket. But everyone can see that they are separate.