John makes a point about the difference between a heart that belongs to God and one that doesn't. The heart that knows God has God and can't help but become more like Him over time. The alternative is a heart that has no place for God and naturally replaces Him with the things that are contrary to Him.
Sin means to miss God's nature in action or attitude.
A relationship with Jesus makes a big difference in the lives of those who receive it.
God hates sin because He knows what it does to those He loves.