What can cause a lack of assurance among believers?

Believers may experience a lack of assurance due to various factors, including sin in their lives, succumbing to temptation, and doubts about their faith. The devil often attacks the believer's assurance, leading them to focus on their failures and weaknesses instead of the grace and sufficiency found in Christ. Additionally, external pressures and challenges may shake one's confidence, causing them to doubt their salvation or relationship with God. This underscores the necessity for believers to continually look unto Jesus and rely on His promises for comfort and assurance.