Why do believers struggle with sin?

The struggle with sin is a reality for all believers, stemming from the remnants of the sinful nature that still resides within them. As articulated by Job, believers recognize their sinfulness and the consequent war within them between the 'old man' and the 'new man.' Although they have been regenerated and are being sanctified, the presence of sin remains a source of struggle and doubt. This internal conflict prompts the continual need for reliance on Christ as their mediator, who not only justifies them but also empowers their battle against sin through the Holy Spirit.