I also noticed that this was a major topic for the early church. However, I saw a different heart behind how it was thought about and conveyed. For one, I noticed that some Anabaptists almost have a fear of the world, that it is by separating from it that they protect themselves, their family, and their church. They have a mindset that separation is important because we cannot become like the world, or else we won't be a Christian.
On the other side, I noticed that the early church had a mindset that we're a Christian, and so by living in accordance to the law of Christ, we become separate from the world.