What then shall we say to these things? If God is for us, who is against us? (Romans 8:31)
If we are going to overcome, we must begin inside. God always begins there. An enemy inside the fort is far more dangerous than one outside.
Scripture teaches that in every believer there are two natures warring against each other. Paul says in his epistle to the Romans:—“For we know that the law is spiritual: but I am carnal, sold under sin. For that which I do I allow not: for what I would, that do I not; but what I hate, that do I. If then I do that which I would not, I consent unto the law that it is good. Now then it is no more I that do it, but sin that dwelleth in me. For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not. For the good that I would I do not: but the evil which I would not, that I do. Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me. I find then a law, that when I would do good, evil is present with me. For I delight in the law of God after the inward man: but I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.” Again, in the Epistle to the Galatians, he says: “For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.”
When we are born of God, we get His nature, but He does not immediately take away all the old nature. Each species of animal and bird is true to its nature. You can tell the nature of the dove or canary bird. The horse is true to his nature, the cow is true to hers. But a man has two natures, and do not let the world or Satan make you think that the old nature is extinct, because it is not. “Reckon ye yourselves dead”; but if you were dead, you wouldn’t need to reckon yourselves dead, would you? The dead self would be dropped out of the reckoning. “I keep my body under”; if it were dead, Paul wouldn’t have needed to keep it under. I am judicially dead, but the old nature is alive, and therefore if I don’t keep my body under and crucify the flesh with its affections, this lower nature will gain the advantage, and I shall be in bondage. Many men live all their lives in bondage to the old nature, when they might have liberty if they would only live this overcoming life. The old Adam never dies. It remains corrupt.“From the sole of the foot even unto the head there is no soundness in it; but wounds, and bruises, and putrifying sores: they have not been closed, neither bound up, neither mollified with ointment.”
A gentleman in India once got a tiger-cub, and tamed it so that it became a pet. One day when it had grown up, it tasted blood, and the old tiger-nature flashed out, and it had to be killed. So with the old nature in the believer. It never dies, though it is subdued: and unless he is watchful and prayerful, it will gain the upper hand, and rush him into sin.
From “The Overcoming Life” (D. L. Moody)