We are sure, that when the Lord delivers us from the guilt and dominion of sin, He could with equal ease free us entirely from sin if He pleased. The doctrine of sinless perfection is not to be rejected, as though it were a thing simply impossible in itself, for nothing is too hard for the Lord, but because it is contrary to that method which He has chosen to proceed by. He has appointed that sanctification should be effected, and sin mortified, not at once completely, but by little and little; and doubtless He has wise reasons for it. Therefore, though we are to desire a growth in grace, we should at the same time not be discouraged nor despond, because we feel that conflict which his word informs us will only terminate with our lives.
– John Newton, Letters of Newton, p. 70.