Answer: The imperfection of sanctification in believers arises from the remnants of sin abiding in every part of them, and the perpetual lustings of the flesh against the spirit; whereby they are often foiled with temptations, and fall into many sins₁, are hindered in all their spiritual services₂, and their best works are imperfect and defiled in the sight of God.₃

1. Romans 7:18; Romans 7:23; Mark 14:66-72; Galatians 2:11-12
2. Hebrews 12:1
3. Isaiah 64:6; Exodus 28:38

For further study see:

1. WSC35: What is sanctification?
2. WSC 36: What are the benefits which in this life do accompany or flow from justification, adoption, and sanctification?
3. WLC 75: What is sanctification?
4. WLC 77: Wherein do justification and sanctification differ?
5. WCF CHAPTER 13: Of Sanctification