These men are dangerous because they came out of Christian churches.  They once, apparently, even fellowshipped with the apostles.  But John would not have us think that these were once real Christians who are now “fallen from grace.”  They may have once companied with the apostles and the Christian church, but they were never “of us.”  Their hearts had not been renewed by grace, and they had not been joined to Christ in vital saving union.  They were religious, but still lost.  Religion to them was a way of gaining popular applause, or perhaps of making money.  The root of the matter was never in them.  John says that the way we can know this is the case with these apostates, who are now teaching blasphemous doctrines against the person and work of Christ, is that they departed from the apostolic teaching and the community of the faithful.  Had they truly been God’s children, they would have continued within the bosom of the church, and in line with the apostolic doctrine.

There are times when religious institutions must be forsaken, and a new line of work taken up.  This occurs when a professing church departs from the doctrine of the Word of God, and itself becomes opposed to the truth.  The protestant Reformation is perhaps the greatest example of this.  The men whom John writes, however, departed from the truth, and separated themselves from those who embraced the truth.  Thus, it was manifest that they were never truly members of Christ’s body.