21 I have not written unto you because ye know not the truth, but because ye know it, and that no * lie is of the truth.
John is not writing to the believers at the Church here
because he thinks they do not know the truth of the Word of God. He is writing unto them to encourage them, to cause them to study the Word and to know what to compare those who come among them to what the WORD has to say.
That is the STANDARD to compare and judge by......
John is encouraging the saints of God....