From today's devotional
Here, John was attacking the second claim of the false teachers: that people had no natural tendency toward sin, that they "[had] no sin,'' and that they were then incapable of sinning. This idea is at best self-deception and at worst a bald-faced lie. The false teachers refused to take sin seriously. They wanted to be considered Christians, but they saw no need to confess and repent. The death of Christ did not mean much to them because they didn't think they needed it. Instead of repenting and being purified by Christ's blood, they were encouraging sin among believers. In this life we are always capable of sinning, so we should never let down our guard.
