The believer has two natures and not one. All teaching that sets forth the [old] nature improved is false. The old man in the saint is always bad. While we are living men here below, the will of the flesh is opposed to God; there is that which God does not improve, and which does not in the least degree admit of improvement.
Our old man is not extinguished, but crucified. "Let not sin therefore reign in your mortal body." It is not gone, but the allowing of it to reign over you is exhorted against. It is like a wild beast which you are to keep under lock and key; but the wild beast does not become tame by merely locking it up. And the overlooking of that point quite accounts for a Christian's falling into what is wrong. He sins when he is careless or off his guard.
There is in the believer, on the one hand, that old nature of the flesh which is always prone to evil; and, on the other, the new man, or the new nature (that which loves God and His will); and it is in virtue of this that man is said to be sanctified. He has got a nature he never had before. He is set apart to God and, being brought by faith under the power of Christ's work, he is said to be washed, sanctified, justified.