Sorry for my lapse, folks. My technical obstacles have abated for the moment, so I'll slip this in...
John’s Gospel and his First Epistle are about two different topics. The Gospel is about receiving eternal life; the Epistle is about manifesting eternal life, and how apostasy is the pathetic manifestation of darkness and hatefulness.
Both the Epistle and the Gospel are centered on absolutes. They both paint their topics in black and white terms. In John’s Gospel, individuals have either received living water, the gift of eternal life, or they haven’t. There’s no middle ground. In John’s first epistle, one either knows Him or they don’t, they are either of God or of the world, again without any middle ground: to love the world is to fail to love God and the brethren.
But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.
It is as if a fine skier is asked to teach total beginners to ski. The athlete teaches them, “There are only two groups of people in the world, people who ski and people who don’t. It’s as simple as that.”
But the same teacher may teach an advanced ski seminar. This time he may say, “There are only two types of people in the world—elite skiers... and everyone else!!”
Both statements can be true without contradicting the other. That’s what’s happening in John and First John.
John is written to total beginners—unbelievers—but John’s First Epistle is written to fully competent believers who as a group are enduring the doctrinal threat of apostasy.