John assumes the perseverance of the saints: if any leave the company of God’s people and return to the world, the explanation is not that they have lost what they had, but that they never truly belonged to Christ in the first place, for God’s true children cannot fall away and will no doubt continue to walk in the truth to the end.
Sadly there are today many churches where the reprobate do not need to go out, for in these churches there is little attempt to set the standards of Scripture before the people, and they can continue there without any discomfort.