I John 5:16 often raises questions about sin and its consequences. This verse is about more than appears on the surface, and holds out hope for backsliders.
It seems that some sins should be worse than others in God's eyes. Though all sin merits the death penalty, some sins carry greater consequences and penalties.
Sometimes Christians fixate on past sins. But we cannot experience the joy of salvation while obsessing on past sins. Christ's blood covers sins repented of.
Salvation is an ongoing work of God, obligating us to walk in the Spirit. If we do, we will be not captivated by the lusts of the flesh.