1 John 2:2121I have not written to you because you do not know the truth, but because you know it, and because no lie is of the truth.
John writes to remind his readers of the basics of the gospel message using a form of double negative at the beginning of the verse to emphasize the latter part of the verse. The aorist tense of have written could refer to the essential truths stated up to this point in the letter. They should already have known about what he has written because these truths are the foundation of faith in Christ. The problem is that they, like us, forget the essentials. Our attention is drawn back to the things of the world through the deceit of the enemy. We do not confess our sins (1:9) but continue to walk in them until they no longer seem like sins. We justify our actions, so much so, that we begin to think that there is nothing wrong with us and that we are not sinning (1:10). We need constant reminders of the way of the truth.
This verse hints at the big challenge: the lie. Many lies are around us, offering empty promises that bring only temporary satisfaction. We can remind ourselves of the truth through many ways: reading the Bible often, committing it to memory, finding ways to live it out; spending time with others who are on the same journey; spending time alone with God; meditating upon the ways of God; singing or listening to praises, and so on. Our lives are like a battery: if we do not keep it filled up with the truth, the world will suck the life out of us until we are depleted and confused.