When Secret Service agents train bank tellers to identify counterfeit bills, they show them both fake money and real money, and they study both. To detect a counterfeit problem, they must look for the differences in the genuine bill compared to the counterfeit—and not the similarities.
In 1 John 2, the apostle John helps to protect believers from heresy by showing them examples of counterfeit Christians and teachers. One of the signs of the last days is the coming of antichrists (1 John 2:18). Antichrists are those who claim to have His power and authority but don’t, or those who reject and oppose Him and His teachings.
John gave three marks of false teachers who are controlled by the spirit of the antichrists: They depart from the fellowship (v.19), they deny Jesus as the Messiah (v.22), and they draw the faithful away from Jesus (v.26). He encouraged believers to protect themselves against the spirit of the antichrists by depending on the indwelling presence of the Spirit, knowing the truth, and remaining in fellowship with Jesus.
We can protect ourselves from error and deception by knowing the false but relying on the Truth—Jesus Christ.