John's first epistle On THE SIN

 in 1 John 3, the sin which John writes is the sin of disbelief of the gospel doctrine that Christ was manifested in the flesh and the hating of those who bring and believe that doctrine. Of course, ultimately the sin John writes against is the sin of unbelief. No questions. Of course, not believing the gospel promise is sin and the result of not believing is that one hates those who believe the gospel. Likewise, those who do believe the gospel should show love towards them that also believe the gospel. Again, what saves is not our love, not even our faith that saves, but what saves is the gospel that is believed.