"Children, it is the last hour; and just as you heard that Antichrist is coming, even now many Antichrists have arisen; from this we know that it is the last hour.  They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, in order that it might be shown that they are not of us.  But you have an anointing from the Holy One, and you all know."

To further John's admonition for fellowship, he points our attention to those that have left the collection of Christ followers, to follow their own hearts.

His description of them is not kind at all.

Their are directly opposed to Christ.  How do we know, John asks.  Because if they were following the same Christ, they would have followed with Christ's people.

This is more a condemnation of lifestyle than of these specific people.  John is saying those that run from fellowship simply because of a difference in opinion are the lowest of the low.  They have no spine, no backbone.  They are without integrity.

Obviously John does not think highly of people that abandon the bride of Christ.

If you seek righteousness - hold tight to others that are seeking righteousness.