Sin is a word that the non-religious generally are not able to identify with, or even acknowledge. Its dictionary definition these days is "the breaking of a religious or moral law"¹ but the "moral" part of that tends to be overlooked. Only the religious can sin, I have heard said, and yet of course that isn't true. We all live by a moral code, usually a blend of our own values and those of the society in which we live. When we do things that break our own moral code, lie, cheat, steal, or in any way find ourselves out of integrity, it's reasonable to name that as a sin, and not get caught up on the religious connotations of the term. Why is this useful? For exactly the reason that John gives: if we claim to be without, or immune from, sin we deceive ourselves, which can only take us further out of integrity, and block us from self-correcting. The greater the denial the more unlikely the healing. Sin embraced becomes a tool for self-reflection, for introspection, for confession and finally for renewal.²

¹ Cambridge Dictionary: sin
² I wrote more on sin here: Embracing Original Sin