If God were to forgive sin apart from the atonement of Jesus, it would follow that wronging another (which every sin does) is not sufficient to warrant punishment. If this is the case, then it follows that God may allow one person to be harmed at the hands of another without requiring the perpetrator to endure an equivalent harm. If that is the case, then it follows that God is treating equal things (human beings) unequally, which is irrational and thus contrary to his character. Thus, God’s nature requires that every sin which harms another be dealt with by afflicting the sinner in question with a harm equivalent to the one he brought about. Because of this, the only way for God to consistently withhold punishment from anyone is by establishing a framework within which sinners can be legitimately treated as though they were not sinners. How this is accomplished is discussed elsewhere on the site.