A better question might be whether or not lying was always considered a sin? You might think so judging from the way Christian apologists scramble to explain the passage in John. Actually the text is very ambiguous.  It’s hard to see what the point of the story is in the first place.  It’s possible that the original text is corrupted here and the translators make due best they can.  I’ll have to dig out my copy of Raymond Brown’s commentary on John.  He’s considered the guru of Johannine studies and it will be interesting to get his take.   

We’ve had this conversation before, but my problem with your theory is that it presupposes we have access to the inner motivations of these ancient writers which we do not. You may be correct that Paul was being duplicitous but we have no real reason to think so.  I suspect he was one of a long line of deluded but perfectly sincere religious fanatics (like Jesus probably was) and it’s not a lie to tell something you think is true even if turns out to be false.