We see immediately that this is a letter of warning. The believer is born again into a spiritual war zone. The powers of darkness are not idle, but begin their assault the moment a person turns to the Lord and through temptation constantly try to draw us back into sin. John is aware that some have not truly repented and never really left the darkness despite their claims. Our conduct verifies our claims.
John’s zeal for the truth does not allow him to be indifferent to error. Those who claim to love God and yet do not defend his word, or claim that it is unloving to criticise false doctrine or false practice, show that they do not truly know the Lord.