So, if belief in Christ and His propitiation is the magic bullet in relational love, why is there a commandment at all? Isn’t He asking us to do something? It is a command, right? Yes He is. He is asking us, in our faith, to walk in the light. He is asking us to admit our sin and shortcomings. He is asking us, in our belief in His propitiation for all of our sins (1 John 1:7), to confess our current existential struggle and apply His blood to our present fears (1 John 1:9). In doing so, we walk in the light with one another, and the result is love. When we temporarily forget that His blood is enough for my current sin or my brother’s current sin, we walk in the present in a season of darkness, and the judgment of good and evil begins to reign over our experience instead of grace. When we go back to the light and confess, in effect we return to the singular command to believe in Jesus His Son and to love one another. The two come flooding back into our lives as one. This does not mean we fell from grace or became unsaved because we didn’t believe for a season. By faith we have entered into this grace in which we stand (Romans 5:1,2), and the blood of Jesus is sufficient for all of our sins forever, past present and future (1 John 1:7). We just forgot that that is the land we live in, we didn’t consider it to be so (Romans 6:11).
http://thereforenow.com/2013/02/1-john-319-24-the-single-commandment-belief-and-love/