John closed with a final admonition. Departure from the true God and His teaching is idolatry. As contradicting God is really calling Him a liar (1:10), so departing from God is really idolatry. Departing from God includes leaving apostolic teaching and practice, behaving as a child of Satan rather than as a child of God.
"False teaching is ultimately apostasy from the true faith.' To follow after it is to become nothing better than an idol worshiper, especially if it is a matter of the truth of one's conception of God. The author is blunt. The false teachers propose not the worship of the true God, made known in his Son Jesus, but a false god--an idol they have invented."183
This verse is a New Testament restatement for Christians of the first commandment God gave to the Israelites (Exod. 20:3; Deut. 5:7).
