2. Sin deceives the mind from seeing the true state and condition of people in the world.

This is the second principal way sin diverts a godly frame of mind. People in their younger days naturally have more vigorous, active, and quick affections. As their minds begin to naturally slow down, the edge and keenness is lost. Nevertheless, unless they are steeped in sensuality or lustful corruptions, they will grow in their insights, resolutions, and judgements. If however, their affections are not educated, they will become foolish old people. It is as if the weak tendencies of childhood are never corrected, and lead to greater, more exaggerated forms of weakness in old age.

Thus the first priority of the Christian should begin with his affections. The deep wound of the conviction of sin and the healing of forgiveness stand out fresh and vivid in the memory and in the spirit of the newly converted. But later, with the decline of the affections, their spiritual sense also changes.

We see this in people who have never developed deep convictions in the ways of God. They become sensitive to sin for a time. They even weep and mourn in their guilt about sin. They make hearty and fervent resolutions against sin. But they are like grass that grows rapidly for a few days, but with shallow root age - quickly fades away. After a while, the more experience they have of sin, the less sensitive they become of its presence, as Eccl 8:11 describes. This happens because the awareness of sin in the emotions never becomes firmly fixed in their mind. Their mind remains preoccupied with the results of sin- sorrow, trouble, grief, fear - that give the mind no rest. But with no real conviction of sin itself, the soul lies in danger of hardening.
~ John Owen