How you,
@John Caldwell and
@CharismaticLady can possibly suppose this is beyond me! What is so difficult about the word 'if'?
If I said your comment made sense, I would be lying, BUT I'M NOT SAYING IT, and therefore I'm not lying.
If John and his readership were claiming fellowship with God and walking in darkness, they would be lying and not doing the truth. BUT THEY'RE NOT DOING THAT.
If John and his readers are walking in the light, the blood of Jesus is cleansing them from all sin. But
if they're not, then the blood of Jesus is not cleansing them from all sin. This is so obvious that I'm embarrassed to be writing it.
These verses are a warning from John to his readers: don't suppose that because you've made a declaration of faith in Christ you can carry on living just as you did before. It's an impossibility because those who trust in Christ have the Holy Spirit which leads Christians to walk in the light. So
if (note the 'if') either John or his readers are continuing to walk in darkness, then inevitably the conclusion is that they are not born again, that their declaration of faith was false and that Christ never knew them (Matthew 7:21-23).
This is the plain meaning of the text. You cannot turn the Scripture into a wax nose to be punched into any shape you want. 'We' means 'we.' If John had wanted to talk about a third party, he would have talked about 'they.'