Hold that thought.
I John ends in 5:21 with a charge, which is the other bookend of our faith: "Little children, keep yourselves from idols." (NRSV) An idol is anything that we put in place of God, be it a stone idol, a set of doctrines, or a search for success. The author of I John warned his readers, that is, to beware of anything within themselves that would prevent them from "abiding in God" (I John 2:28). Beware of our deeply planted human inclination to turn anything holy into something we own and have control over—and to turn those things what we desire into our vision of holiness. The greatest danger to our abiding in The Light, that is, is our own tendency to want to own, control it, and have it be what we want it to be.
We are called upon to fill in the blank between 1:5 and 5:21, that is to discover what it means to abide in The Light, which has a good deal to do with loving one another (4:7). Indeed, Love along with Light is another metaphor for God. So: God is The Light. God is Love. Abide in The Light. Love one another. And don't turn any of this into a process that we think we own and control. At the end of the day, faith in God, who is Light and Love, is not something we have so much as something that has us. And that's tough, because we don't like to be had. We just plain don't, which is why 5:21 may be the most important single verse in the whole Book of I John.