John is so determined to drive home this point that he admonishes them to let this truth abide in them, the doctrine of the apostles which they had heard the very first time the Gospel was preached in their hearing.  Those who depart, he has already said, show that they were never of us.  But for the true children of God, he wishes them to know that, when they abide in the message of grace that they have heard from the beginning, they are continuing in the Son and in the Father.  They continually enjoy that blessed fellowship he spoke of in chapter 1.  Such a privilege can only be maintained by abiding in the truth.  When we abandon God’s truth for error, then we lose the privilege of fellowship with Him, and descend into a pit of darkness.

John reminds them that the promise which was first preached to them is the hope of eternal life.  It is that gift of God that Jesus spoke of, when He declared that those who believe on the Son of man, lifted up on a cross even as the brazen serpent was lifted up in the wilderness, will have everlasting life.  It is the message of grace obtained through faith, of the new birth, of reconciliation with God.  In short, it is the Gospel.  Those who believe on Jesus Christ, Who is the true God and eternal life, have eternal life as a gift from God, rather than suffering the wages of sin, which is eternal death.