Sanctification, again, is a thing which does not prevent a man having a great deal of inward spiritual conflict. By conflict I mean a struggle within the heart between the old nature and the new, the flesh and the spirit, which are to be found together in every believer (Gal. 5:17). A deep sense of that struggle, and a vast amount of mental discomfort from it, are no proof that a man is not sanctified. Nay, rather, I believe they are healthy symptoms of our condition, and prove that we are not dead, but alive. A true Christian is one who has not only peace of conscience, but war within.
-J. C. Ryle (H/T: John Piper)