This going out from the church of God is not something trivial, but is a turning away from the truth altogether. These apostates were uncomfortable being in fellowship with those whose lives were governed by the Scripture.
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.
Why did those who are not converted join with the people of God for a time? They had some spurious reason for doing so, but their adherence could not survive the rigours of the Christian life and by and by they were offended. John perhaps has Judas in mind.