Whence ariseth the imperfection of sanctification in believers?

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,[1] are hindered in all their spiritual services,[2] and their best works are imperfect and defiled in the sight of God.[3]

1. Rom. 7:18, 23

   For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out.
   
   …but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members.

   Mark 14:66-72

   And as Peter was below in the courtyard, one of the servant girls of the high priest came,  and seeing Peter warming himself, she looked at him and said, “You also were with the Nazarene, Jesus.” But he denied it, saying, “I neither know nor understand what you mean.” And he went out into the gateway and the rooster crowed. And the servant girl saw him and began again to say to the bystanders, “This man is one of them.” But again he denied it. And after a little while the bystanders again said to Peter, “Certainly you are one of them, for you are a Galilean.” But he began to invoke a curse on himself and to swear, “I do not know this man of whom you speak.” And immediately the rooster crowed a second time. And Peter remembered how Jesus had said to him, “Before the rooster crows twice, you will deny me three times.” And he broke down and wept.

   Gal. 2:11-12

   But when Cephas came to Antioch, I opposed him to his face, because he stood condemned. For before certain men came from James, he was eating with the Gentiles; but when they came he drew back and separated himself, fearing the circumcision party.

2. Heb. 12:1

   Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,

3. Isa. 64:6

   We have all become like one who is unclean,
   and all our righteous deeds are like a polluted garment.
   We all fade like a leaf,
   and our iniquities, like the wind, take us away.

   Exodus 28:38

   It shall be on Aaron’s forehead, and Aaron shall bear any guilt from the holy things that the people of Israel consecrate as their holy gifts. It shall regularly be on his forehead, that they may be accepted before the Lord.

Westminster Larger Catechism, Question 78