If you understand what the parable of the wheat and the tares means, it helps you to see what John is teaching ‘the little children’ in I John. There is going to be infiltrators into the remnant church in the last days.
Peter and Jude describe what these children of Satan will be like on the inside. John is teaching the newly born again what to look for on the outside.
The difference will be in the fruit. Wheat and tares look similar until they bear fruit. Then they are easily distinguishable.
When John writes “he that doeth righteousness is righteous, even as he is righteous,” he is describing the fruit of the wheat.
When he writes “He that committeth sin is of the devil,” he is describing the tares.
He makes this plain when he writes: “In this the children of God are manifest, and the children of the devil.”