Q. Whence ariseth the imperfection of sanctification in believers?
A. The imperfection of sanctification in believers ariseth 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 sins, are hindered in all their spiritual services, and their best works are imperfect and defiled in the sight of God.
Q. May not true believers, by reason of their imperfections, and the many temptations and sins they are overtaken with, fall away from the state of grace?
A. True believers, by reason of the unchangeable love of God, and his decree and covenant to give them perseverance, their inseparable union with Christ, his continual intercession for them, and the Spirit and seed of God abiding in them, can neither totally nor finally fall away from the state of grace, but are kept by the power of God through faith unto salvation.
(Questions LXXVIII and LXXIX of the Westminster Larger Catechism.)